The strategy of Trumping In is the exact opposite of Pulling Trump. The main idea behind Trumping In is to allow for a partner to ruff a trick. Generally, the trump maker should pursue Pulling Trump. In some cases Trumping In is the only way to win.
Let’s look at an example game to demonstrate when to Trump In.
South has named hearts trump. The grand scheme for South is to ruff East’s lead with the A, pull trump with the J, and follow through the A. If needed, South may depend on North ford help.
A 10 9
East leads off with the K. South follows suit with the A. West trumps with the Q, and North follows suit. Obviously, plans did not go right for South.
East leads the A, and play proceeds with the 10 and the 9. South trumps with the A. Currently, South is holding J and 2 losers.
South desperately needs for North to win a trick. Leading the J would pull the remaining trump from East and West, but it could also pull a winner from North. South needs to keep as much winning material in play, and allow for North to Trump In.
South leads the 9. Play proceeds with the K, the Q, and the Q.
West then leads the 10 with North following suit. West ruffs with the 9. With the trick score already 1-2, South must play the J if there is any chance of saving the game.
South plays the 10 followed by West’s 9. Finally, North is able to Trump In.