Color Blocks Escape

Magician opens the box and shows different color blocks that hold by a pin to audiences. Audience chooses 2 color blocks and takes them out.Magician puts back the 2 color blocks and closes the box. Pin is put back into box such that the blocks are held by the pin.
Magician applies magical power, and open the box.
Surprisingly, 2 selected colors block drop out from box without taking out the pin.