How do I perform the infinite Ice Climber grab?

In a video I saw a long time ago, I saw Popo and Nana (Ice Climbers) grab an enemy, and infinitely hit them. The other player was unable to escape, and had to sit there watching as his damage racked up. The Ice Climbers then Up Threw him off the stage for a KO. How do I replicate this and use it to my advantage?

If I remember correctly, the video I watched was Melee at a tournament.