To edgeshot, you basically need to think of it like you try to speedshot at the very edge of the platform, it's more or less the same timing.
I don't think there is like a special edgeshot that's extremely hard to do, most of my edgeshots had very similar speeds, and I believe what happens is edgebug first, then rocket right after. Obviously I might be wrong tho.
The main thing about it is you get way more horizontal speed (1 edgeshot after a ctap can put you @ over 1100 u/s ->
link), which is usually more useful than vertical. Also you can edgeshot on platforms covered with trigger_teleports, helpful with beating some demo maps as solly.