21's seem to jump start gains again for quite a few folks. 21's are also known as 7's and maybe other names...not sure.
Different ways to do 21's, but all can work well. 2 or 3 cycles should do the trick as the only bicep exercise for that workout. Think of 21's as something akin to Running The Rack, where super intense movement is required, and at a fast pace. Both styles can be killers on the biceps. Racks can be used for delts also. Might do 21's a couple times a week, for a few weeks and than go back to your regular arm program. And than back to 21's again.
Just my view: If anyone needs to do "Finishers" at the end of a workout, their workout wasn't well planned in the first place. Really not needed if thought, and good effort, is put behind any training session.
Good Luck