Zap Jump is an advanced technique used for Lucas. It was discovered by Christopher Daniel Winans and shown for the first time in this video. This technique doubles Lucas's second jump.
To execute this move, you must use PK Fire within one frame of your double jump. The timing takes a bit of practice, but you basically hit the jump right before you use PK Fire and Lucas will shoot up quite a long distance. This can be combined with Lucas' other techniques such as B-Sticking and Magnet Pull for an impressive recovery. To make the execution of this technique easier, it is advised to set the L-button (or R-button if you roll with the L-button) to jump, so you do not have to claw the L and B buttons at the same time.
PK Jump
There is also an interesting effect when Ness' PK Fire interacts with his second jump. When PK Fire is executed at the same time as his second jump, he shoots backwards and up a bit while using his PK Fire. While this has no use in recovery, it makes a great approach due to the way he shoots his PK Fire. It also extends PK Fire's range greatly.
How it works
Members of Smashboards have discovered why this happens. Normally on Lucas's second jump, an upward force propels him upwards, but the ascent is hindered by a downward force right after this. PK Fire somehow cancels the downward force, causing a huge, fast jump.