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For example, bluffs are much stronger in pot-limit and no-limit games, because your opponent will have worse pot odds, in addition to the threat of larger bets in future betting rounds if there are any.
Bluff much less in high-low split games—some very weak hands will call hoping for half the pot, and the likelihood of splitting the pot greatly reduces your pot odds in any case.
If their bluff fails and they are called, they still might be dealt a spade on the final card and win the showdown (or they might be dealt another non-spade and try their bluff again, in which case it is a pure bluff or stone-cold bluff on the final round rather than a semi-bluff).