The short answer is they are bookmarks. The longer and better answer is that "instas" are a very useful tool that prevent you from being shot by dudes camping a station.
Before tethering on citadels, this strategy was crucial for survivability. For citadels, the tether will keep you invulnerable - even while aligning for warp - unless one of these is true:
It is still a good idea for any NPC station you use often, such as Jita or Amarr trade hubs.
Every station has a certain "docking radius". If your Overview shows that the distance to the station is "0", then you are within the docking radius and are able to dock immediately, even if it looks like your ship is still quite far away from the station.
Warping to a station at zero or clicking on "Dock" does not guarantee that you will land within that radius. Depending on the direction from which you warp, you might land up to 2,000m away and you will need to 'slowboat' to reach the docking radius and dock. That makes you a juicy, killable target. To avoid that happening to you, we're going to make an "Instadock" bookmark that sits right on top of the station and allows you to warp and then dock straight away.
To set up an Instadock you must be within docking radius of the station (i.e. at 0km distance). Press Ctrl+B (or open People and Places window and click "Create Bookmark") and save a new bookmark. This bookmark is now your Instadock. (If there are multiple stations in use in a system, give the bookmark a helpful name so that you know which station it relates to.)
Note that you can place the bookmark within the station, but every time you warp to something you land slightly off the bookmark. While the idea of having a BM within the station is nice, if you land within the actual station model, you will bounce out of the station, so the best way to set your instadock is to set it within docking radius and outside the station.
As the instadock is only dependent on the docking radius, it can be quite useful to make more then one, especially in nullsec, where the physical undock of the station will often be bubbled in order to camp people in. Make a second instadock on another side of the station, within docking radius to avoid bubbles covering the physical dock of the station (obviously this is not necessary in low or high sec, where bubbles cannot be deployed).
An Instaundock bookmark is one that you can warp to immediately after undocking, without needing to align or accelerate.
In order to understand undocks we need to talk about how warping to something works first. To enter warp, you need to be aligned within 5° of your target and your velocity has to be at least 75% of your maximum velocity (including any propmod that you currently have active). If these two prerequisites are met, your ship will enter warp.
As you undock from a station, you will leave the station at 100% of your maximum velocity within a random cone 5° off the invisible line marking the middle of the station undock location.
So, if you make a bookmark over 150km from the station that is on that invisible line, then you meet the requirements (since you are within 5° and moving at at least 75% velocity) and can instantly warp to that bookmark, your instaundock.
As the total undocking cone is 30° (15° from the middle axis)Trinity 1.1 patchnote , if you undock and move, let's say 500km away, you might move out of the instaundock BM's cone. For example: you undocked 3° upwards as you made your undock, but now you undocked 4° downwards. You are therefore not within the 5° limit you need in order to instantly warp to the bookmark.
Since your ship has to realign before it can warp to the bookmark and this will take time at full speed, a bad instaundock like this will get you killed.
So basically you want to have your undocks as close to that imaginary line as possible to be within the 5° every time.
There are two ways of doing this: a bad one and a good one.
The bad one is undocking and checking if your ship changes direction, by watching the trail of your thruster. If not, burn away and make the undock. This will take a lot of tries and therefore a lot of time.
The faster way is to pilot your ship manually along the line in order to make the undocks. Place your ship within the physical undock and adjust your height and vector using the tactical overlay. The overlay is centered in the middle of your ship. To get the right height, zoom out, and view your ship from the side, until the rings of the tactical overlay merge together to a single line. You need to adjust your point of view until this happens. Now the point where the "line" cuts the station marks your height.
You need to know where the imaginary axis is. If you take a look at some Minmatar undocks for example, it is in the lower third of the physical undock. Just aim with the tactical overlay line as close to that point as possible. Yes, you kinda have to eyeball it.
Now move your ship as close to the middle of the undock as possible. Use the numbers from the overlay to move along the imaginary axis. Move your point of view behind your ship until the rings merge together again. Aim straight outwards and doubleclick on the overlay "line" in order to move out along said axis.
I recommend at least offgrid. In lowsec this is roughly the distance at which the stationguns disappear. Better add some couple 100 km, just to be sure. Your absolute minimum distance should be 200km so that you can drift for a little while before you need to warp.
Better make longer distances, as some dudes tend to use the way described above to make their own, which might, if done correctly be within point distance of your bookmark. Also a lot of dudes tend to warp behind you and kinda guess your distance. If they manage to land on grid with you, this bookmark is now compromised, as they know your distance and are able to warp to you, if you keep using it. (We killed a test dude in YZ-LQL quite often because we guessed his 500k undock.)
So use longer distances, a couple 1000km, to ensure your instaundock is off-grid; and make more than just one so you have options. Do not always use the same bookmark. You do not need to go full lumpy, but keep in mind that you need to set up your undocks only once.
Also avoid probers by immediately warping off once you land on the undock.
Yes. Every time you undock, you get a 30 second invulnerability timer. During that time you also have no collision model. Some station campers tend to hang around a little distance away from the undock in order to block your way, so it can be possible for you to bump into them and lose your alignment as you break your invuln timer and become collidable again when you warp to the bookmark. You have a full 30 sec without being bumpable to get past them, but take care, as you might move out of the docking radius while doing so. If you do this and use a bad instaundock you are in big trouble.
If you just want to take a look at what is outside the station and/or grab a dscan for your FC, stopping your ship (hit the minus on the capacitor thingy, or use Ctrl+Space) will not break your invuln. Dscan'ing also doesn't break invuln. But almost every other action will, including redocking. Make sure that your session change timer fully depletes (a little grey ring on the top left of your screen, 10 seconds) before you try to redock. You will get the timer every time you undock, but also from other stuff, including taking another position within your fleet, moving to another squad for example, and getting shot out of your ship.
Therefore if your ship gets blown up on the undock, you have to wait 10 secs before you can dock, even if you don't have a weapons timer. So warping off is the better solution when something goes horribly wrong.
Take care, fly brave and use instas to be able to ignore the dudes camping our station.