ECB extensions are not updated frame-by-frame, but only at certain keypoints. You can think of them as something similar to video keyframes in h264, if you are familiar with these.
My knowledge about them stems solely from looking at their extensions at different frames, so I can only offer “educated guesses”, but not actual insight into Melee’s code.
This is how I understand it currently: Your basic ECB extension is updated approximately every 10 frames. This seems to depend less on individual actions than on body states like airborne / grounded (hanging on the ledge counts as grounded here, although this is more ambivalent in other cases – giving you your double jump(s) back, but not your pseudo-jump). It was probably programmed like this so that moves that lift your feet upwards (think CF’s uair) don’t instant-land when you drop through a platform and start them immediately.¹
At different points within one update interval, your ECB extension is not constant and still depends on your animation frames assigned ECB extension shifts. But the basic shape will only change considerably when the next update interval starts.
Grabbing the ledge takes exactly 8 frames, except for Link and Young Link where it’s only 4. If you drop from the ledge within 3 frames (after
CliffWait 0, 1 or
2 ), you will still stay within the old update interval and keep the, hopefully beneficial, ECB extension. If your character’s double jump speed accelerates sufficiently quickly, you can rise above the stage before the next ECB update interval starts.
Differently and simply put, you try to stay in each body state for as short as possible (
<10 frames²) so that the game lags behind and doesn’t correct your body extension from aerial to grounded before you go from the ledge above the stage.
Ganon is probably the most bizarre abuser of this exploit. I mentioned a few applications here:
http://smashboards.com/threads/is-there-anything-left-to-discover-with-ganon.335743/#post-16442699
But I didn’t really explain how to get it like this. I still believe that I can explain this much better in a video, but I will post a short analysis later (currently working on it, but I want to complete this post for now, haha).
¹So-called “platform canceling“ does just what the programmers wanted to prevent, as they likely didn’t account for the hitlag delay that also increases the ECB timer and thrusts you into the next ECB update interval. Because aerial ECBs have much shorter downward extension than grounded ECBs (those will usually be kept for 10 frames so that you can pass through the platform), the updated ECB might have you being “above” the platform again, if your aerial’s ECB downward extension is sufficiently short to compensate your falling speed.
²Light orange color indicates estimates without proper testing and verifying from now on. So these 10 frames could be 9 or 14 as well, but if I always run such tests before I post, I won’t write another post before I understand pretty much the entire Melee code, haha.
