Forum:CHOPPER

chopper said that he only needed the rumble ball for one(still unknown) transformation and the "other six" can be done at will, yet in chapter 605 he took the rumble ball to transform into the "new" Guard Point to deflect kraken's attack... did Oda missed that detail?
 * i think Chapter 645, have he answer to your question... THE MONSTERR POINT! Chopper still uses rumble ball to transform him to monster point, but unlike before he's definitely under control during this transformation.

I think Chopper took the rumble ball to boost his transformation enough to protect the whole ship. .