Just like giant sets, with 21's you overload TUT with slightly above average weight. However they are very exhausting wich is why you give up on the abillity to lift really heavy weight afterwards, which is equally important part of the equotion.
Therefore, like some people already mentioned use 21's
- as secondary exercise, do the straightforward heavy bread and butter exercise first
- or use them sparingly, like every 4 weeks, so that you have built more basic strength in the meantime and you can do the 21's with relatively heavy weights and get a real blast from them.
- when you hit plateau