From SmashWiki, the Super Smash Bros. wiki
Tree-grabbing, also known as the Fast Like Tree grab, is a technique in Super Smash Bros. Melee that allows Marth to grab behind him out of his running animation, comparable to pivot grabbing in later installments. To perform this technique, the player must start running (takes 15 frames), hit down to Dash Cancel (up to 6 frames), then dash forward again. After the 3rd frame, quickly pivot the opposite way then grab. The player can use it both offensively and defensively with incredible results.
The name was coined by FASTLIKETREE.
Video of Tree-grabbing