What would be an english translation for "手中法訣"? By "手中法訣", I mean where their hands are rapidly moving in certain patterns in a special movement, but I don't know what's it called (if there's a name for it) nor how to say it english.
First of all, what you are saying doesn't even exists in Chinese. 
Chinese magic emphasis ritual and 心法口诀. It never specifically mention any gesture. Let alone a combination of multiple gestures.
So your translation is determined by what magic system you use.
If your system is based on naruto. I think you already got the answer.
If your system is based on western magic. The I think "spell" is the word you are looking for.