Upon pressing the special button, Samus shoots the Plasma Whip forward a long distance, with small knockback and damage throughout its length, except for a fairly large sweetspot at the tip. The body of the whip can knock the enemy into the tip, which has large knockback (at least as large as Samus' side smash, but with longer range) and is considered her best KO move outside of her forward and back aerials. Does 19% damage (if the opponent is close to Samus because its pushed to the tip, doing two hits), 16% if only the tip hits.
The move is a little laggy after extending, but comes out at about the speed of a smash attack. In the air, it can be used as a tether recovery; it differs from her other tether move, the Plasma Wire, in that it has longer horizontal range, but much less vertical range, giving Zero Suit Samus several options for tethering, depending on whether she is level with or below the ledge. The Plasma Wire can also grapple ledges that are below Zero Suit Samus' level, which can be useful for varying up her recovery. There is a small blind spot in the middle of the Plasma Whip which the opponent is not hit.
 Description from the Instruction Booklet
Attack with a lash formed of plasma. This move can also be used to grab onto the stage edges.