Mario&Luigi is fanmade Mario game
This game seems to use the common jump-on detection algorithm in Mario games: If you are moving downward while hitting some enemy, you jump on it (and don't die unless it is piranha plant or spike top).
Piranha plants don't hurt mario while they are popping up from pipe (while going down they do hurt him).
If you press left+right, Mario instantly changes direction.
If you hit block suitably from below, you go bit inside it and get ejected. This saves 2 frames a pop if performed correctly.
Picking up bonuses inside block:
If you hit block that's ejecting bonus with sufficient speed, you go bit inside block and hit the bonus, picking it up.