Edit: i just tested. I was wrong. So my guess was off and i rather remove it. Rather, what i got to:
1. The effect at the start of the video i am able to reproduce. I first have to activate the FSS, then while the screen still is changing, activate the FSD. Apparently the ship still takes normal input while the FSS is not fully initialized, but as the process of switching to the FSS was started, the FSS will be displayed.
2. If i don't touch the throttle while doing this, i can not trigger the jump any more from inside the FSS.
3. If i move the throttle forward in the same half a second where i can activate the FSD charge while opening the FSS, i do mange to reproduce the sequence of the video. That is, if you pressed a button to leave the FSS just before the jump happened in the video.
So the complete squence is:
- Aim your ship to the destination system.
- Activate FSS.
- While the FSS screen is displayed but not all elements are initialized yet, hit the button for charging the FSD and move the throttle fully forward.
- Wait till the FSD is displayed to be fully charged.
- Leave the FSS.
- Jump happens.
I don't see much use to exploit this for your advantage, so i guess it's not critical. But it's a nice catch.