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DefyPls
It doesn't matter how long you hold the x/y or R/L as long as you press them within 20 frames of hitting the wall, and don't have your tech disabled by earlier R/L input. It's explained in the video too. If you're still inefficient, there are three major aspects I think you might go wrong with.
1) You have to be very close to the wall before you get hit if the hit sends you away from it, o.w. it will be very hard to touch it via [A]SDI.
2) You have to [A]SDI into the wall. So make sure you're holding towards the ledge when you get hit, unless you go for SDI tech, which might be required at high%.
3) You might be hitting the R/L too late. 20 frames is a long window. Whenever I sweetspot, I press R a bit before I reach the wall/edge. So I kind of option select teching before I even know whether my opponent is trying to hit me or not. I don't try to react to a move and tech, because you can't input tech during hitlag unless you SDI into the wall on the same frame.
You can sdi the uair in any direction, and the best direction to sdi to depends on your character and the way the fox is moving. With puff you often sdi up for example, with marth I think side works usually better. And while the hitlag of the 1st hit of uair is 4 frames, you can't sdi on the 1st one (the hit frame), so the window is actually only 3 frames.