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.
Over here, I'm taking an educated guess here but I think you may be focussing on Chinese Taoism/Taoists or something that focusses on "fairy/divine/immortal" (仙) where there is an element of magic/Taoist spells being involved that are composed of rituals, symbols and hand movements.
A less literal translation would be "invocation" as in the invocation of a spell or magic.