I saw a video clip by where you mention that it is permitted to do sajdah on the back of your hand. Is it applicable only if the sajdagah/mohr breaks or at other times when it is not available?

It’s only permissible when you don’t have anything to pray on that is allowed during salah, for eg when Turbah is removed or you lose it during salah.

Under normal circumstances you’re not permitted to use back of hand