Hello. The tutorial said that weapons can be attached using “veld”. But if I want to detach one hand in some frame, is it possible? Maybe I should use a different constrain: “distance” “attach”?
Hi @Arat_Arat and welcome to the forums! 
Correct, the Weld Constraint cannot be animated. The animatable version is the Attach Constraint, try that and post here if you have any further questions.