Instant while running (iWR) is a technique to do running moves at close range(closer than range at which your character starts running by just pressing f,n,f).
Despite the name, iWR has nothing to do with being fast.
The key to consistently do iWR moves at point-blank range is to press the attack button one frame after the third f input.
The input overlay was modified to show inputs on per-frame basis. As you can see on one of the frames of the video:
Dragunov did iWR2 at very close range but I did not enter the 2 input one frame after the third f. I only entered 2 two frames after the third f.
The iWR technique makes use of how the game goes about moving you to a running state and stopping you from running when you hit the opponent with your dash. Rather than stopping you from doing a run at all, the game always switches to a running state after you press third f, and only switches your character back to a standing state after it hits the opponent with its model, so there's always at least one frame where you're in the running state.
Running moves are powerful. For example, Kazumi's wr2 deals 25 damage with knockdown and wall splats. The only other safe mid she has with better payoff is f,F+4, which is slower, shorter range, easier to step, and worse on block. She also has wr1 which is +7 on block and deals even more damage.
Rather than making running moves fast, the purpose of iWR is to make them possible at range that is less than range at which your character starts running on its own after entering f,n,F. They're at their best at around range 2, but it's possible when you go for one that your opponent does a dash (e.g. a dash block) at the same time. If this happens and you don't do iWR, you don't get your running move.
Even when input quickly, running moves are slow—the input still adds 5 frames minimum to the startup, and more realistically 10+ frames.
iWR does make running moves slightly faster when you press the attack button at the first frame of your run compared to 2 or 3 frames into it as one might do often.
Overusing iWR at close range without a read on your opponent's timing is good way to eat a counter hit, but having the option to use running moves in this situation is also useful, especially when at the wall.
You can buffer f,n inputs of iWR. This makes it faster and thus more of a threat when combined with a big frame advantage. Doing this is a bit harder as now the second f input should only be entered after your character recovers. If you press second f too early(before your character recovers) you'll get the wrong move, and later will be slower.