Basically, instead of hanging from a fingerboard, you get (or make) a hold-y type thing, attach weights to it, then pick it up off the floor (with your arm down by your side).

So you don't have to put up a fingerboard in your house, you can build up the weight from a very low level (which is hard to do with a fingerboard without a pulley system), and some people report that it's easier on their shoulders.

https://climbingtogetherandotherfunadventures.wordpress.com/2016/08/19/no-hangs/
https://www.reddit.com/r/climbharder/comments/4ojsfx/no_hang_apparatus/

Trains isometric strength, which is what you need for climbing. And you wouldn't have to use your thumb!