usually it stops when there is more than one move that are good. there is some cutoff based on the computer scoring of the moves as to when it should be considered the "best move", but i don't remember what that number is.
Tactical Trainer Suggestion
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Tactics trainer problems are not only required to be not ambiguous, they should also be conclusive.
When you encounter a problem that seems clearly too short to be conclusive enough to show the point of the problem, it might have to be modified or deleted by a tactics trainer moderator. So you are welcome to report such problems, or at least to leave a comment in its comment section.
Not sure if this is the right place to post this...
Anyway, I feel that sometimes the tactical trainer stops too early. Especially when you fail a puzzle and you see the correct move you normally have to spend another minute or so realizing why it was the correct move (eg forced mate which the system doesn't bother to play out etc).
I think it'd be a nice feature for the system to "play out" the next few moves when there is a forced situation that the user may not have spotted.
Cadaz