Yellow Switch Palace is a secret level in VIP1-WORLD1, as well the first switch palace found in the game.
Now you could just head straight to the switch, but all switch palaces have a bonus way of earning 1-ups.
For this one, you must pick up one of the throwable blocks at the beginning, drop it towards the purple Koopa, and let him kick it. The block will hit into the enemies ahead, scoring a bunch of lives, so you must follow it so it doesn't despawn. At the end of the block's journey, two purple Koopas will kick it to each other. Kill the left Koopa, and the other one will kick it to a block that contains one more 1-up.
When you hit the yellow switch, you will be greeted with this message: