find the worst move

I post this because it is the first time I see castling as a possibility to make something similar to a discovered check

the worst move is 1.Rb8+ losing the rook with no compensation.1.Kxd2 atleast wins a bishop in return.

if this is the first time you see such a thing,you need to work on your tactics a lot.

Well actually...this might not be a bad way to go over games from time to time. Everybody is always looking for 'Best move "this" and 'Best move "that". Half the issue with players is being unable to look at a move and say it is inherently worse than another.

Thus I think it would be better to say, "Out of these three [good] moves, what is the worst to make and why?

1. Rb8+ is not the worst move - it is an illegal move.  White is in check.

The trouble is if you don't capture the B that you are down a piece anyway.

I guess black must have just captured on d2 to lead to the diagrammed position?

correct,i overlooked white is in check.

for a second I thought "this is useful to know", but then I noticed this isn't applicable to other pieces (as it will prevent the castle), this trick is tied to a rook on b2/b7 queen side or h2/h7 kingside (although I can't think a single reason why RxR wouldn't be better).

edit: oh look, here is a game where this is used http://www.chessgames.com/perl/chessgame?gid=1104131