A method of leaving only one player in the position of guarding a particular suit, thus increasing the chance of executing a successful squeeze. Technically the more correct term is isolating the guard.
You and North do well to reach 7♠ played by South. You win the club lead in hand and draw two rounds of trumps with the ace and king. When you discover the 3-1 split, it would be risky to attempt a fourth-round diamond ruff in dummy.
Instead, you play for a squeeze. You want to arrive at a position where only one defender is guarding both red suits. You cross to the ♥A, ruff a heart high, back to the ♠Q and ruff another heart high.
After three rounds of hearts, you can be certain that only one defender can guard the suit. When you cash your last trump and cross to the ♣K, East has no answer.