Not judging anyone who is into this but it seems pretty unsafe to do with implants, especially large ones which are more prone to rupture/leaks etc.
Edit: Well I watched the video and what they were doing was definitely safe, haha. I don't think there was any actual slapping or punching taking place. They were holding the boob with one hand and letting it fall back as they moved their other hand forward. So basically the only contact was when the boob rested back into place