The key point is that pausing can be used to morph much faster than double pressing down.
"R" means holding angle aim before the first down press so Samus has a smaller hitbox for slightly more space to work with.
1) Hold angle aim (up or down) and down while in a crouching position.
2) Pause and jump as late as possible.
3) Release all inputs during the pause screen.
4) Hold jump and down and forward after the pause screen clears but before game control resumes.
See also: Tech Page