rod03801, Pedro Martinez, and Ceiter have done a great job of discussing the Knight's Charge, so I'll just add a few words.

The Knight's Charge is never mandatory.  However, if one of your pieces is in position to Jump one of your opponent's pieces, you must make a capture sometime during that move.  (The one exception is when you have Jumped into your own Castle.  Even if there is an available Jump somewhere, you must leave your Castle immediately.)

Anyway, if a Jump is available, you can accomplish the mandatory capture either by the Jump, itself, or, if one is available, by a Knight's Charge.  When a Jump is available, and you start your Knight Cantering, you must end up capturing with that Knight (i.e., by a Knight's Charge).  So, when a Jump is available, a Knight is not allowed to Canter and then stop its move before Jumping.

I hope that's clear--I'll be happy to elaborate further, if necessary.