When it`s my turn to act preflop, I will either pass and push my cards away from me across the line or call and place my card protector on top of them to indicate that I will be keeping them. This seems to me the most obvious point at which to employ the card protector.
The key thing one must beware of (which I have seen some players do at the table) is to absent-mindedly place the card protector preflop as soon as your cards are dealt and before your turn if you are intending to play but not if you are intending to fold. This is a huge tell for any opponent who is even half awake.