The white card is placed in the box's compartment with the red dot on the door and the door is closed. You tell the audience that you will make the white card vanish. As you say this, you tilt the box so the blue dot door is lower than the red dot door and the white card slides across to the blue dot side of the box. The audience sees the card slide across the gap between the two doors. You then open the red dot door to show the white card gone. The audience yells to you that they want the blue dot door opened. Now you tilt the red dot door side down and they see the card slide back. You now open the blue dot door to show the compartment behind it empty. After sliding the card back and forth a few times and open the doors to tease the audience, you then agree to open both doors at the same time. Both compartments are now shown empty!
A fun trick for entertaining children.