Use it after moving and choosing
First make the normal single reel shift and select a valid three-space pattern. You may then turn your Strike card face down to replace one symbol in that pattern with a symbol already present in one of its other spaces. The card never supplies an extra reel movement.
For example, Cherries–Bell–Cherries can be counted as three Cherries by substituting the Bell. A line of Lemon–Bell–Coin can become a pair by matching one of those visible types. You cannot choose an absent symbol merely to obtain a seal missing from your collection.
Count the altered pattern once
Take the coins and seal earned by the temporary result. Keep the spent Strike card face down so its use is unambiguous. Afterwards the original reel symbols remain where they were; the next player sees the real board, not the substituted counting example.
A natural triple cannot be improved by Strike, so save the card. The practice board identifies a substitution that improves the immediate match. It is an explanation of the rule, not a promise that the same choice is best for every personal collection.
Keeping it also has a value
A player who finishes all six turns with Strike unused adds one point to the final score. This creates a small choice between improving an available pattern now and retaining the card. Once spent, the card cannot be recovered by a later match or by exchanging coins.
If the table is uncertain whether the card has been used, pause before the next move and inspect the player's area. The visible face-up or face-down state should settle the question without reconstructing several turns from memory.