It’s kind of two separate issues. For hypergrav loops, you’re just dashing and canceling with H. Since you want to hit the H nearly as fast as possible (but you don’t want to accidentally plink a dash), dashing with L+M simplifies the whole thing by keeping you from having to hit, say, M+H, lift your finger off, and hit H again. If you play on a pad and use a single button dash on a trigger, it can be hard to be 100% sure you’ve completely released the trigger before you hit H, and, since the game won’t let you input H as a normal when it’s already being held down from a dash, that makes it really difficult (if not impossible) to do.
As far as the Schmidt select goes, basically the deal is that “real” plink dashing would just be two buttons–say L~M. If you hit M one frame after the L, the game’s input leniency turns the now-L+M into a dash while still having given you one frame of startup on the L (I think). So characters who can cancel dashes with normals are basically hitting L, turning it into a dash on the first frame of startup by inputting M, and then canceling that dash into another L, plinking L~M, over and over.
The Schmidt select is different for some reason because it gives you slightly more than a frame to cancel your normal into a dash. I don’t know the reason for it, and hopefully someone’ll give a better answer, but if I had to guess, I’d say that the game gives maybe 2-3 frames (makes a significant difference) during the startup of a normal where it can be canceled into a multi-button dash, which can then be canceled into another normal, etc. As far as L+M+H vs. L~M+H goes…I don’t think there’s a difference in leniency between Schmidt’s version and a three button plink, but it definitely takes less dexterity. I guess his point was that, if you use 2 buttons with his mapping, you’re getting the simplicity of traditional plinking with the leniency of the slightly more demanding 3 button plink. Also, the main thing is that the increased leniency over the A~A method lets you easily option select a throw because you’re doing direction+H every time you dash. That being said, you want to plink H~L+M if you’re not doing the Schmidt select.
TL;DR: Doesn’t do anything for hyper grav loops, but don’t airdash with M+H. Dashing option selects a throw and it’s more braindead than 3 button plinks.
EDIT: Misread your post. No, don’t use L+M+H for hypergrav loops, just dashing (if you don’t want to try 3 button plinks).