If you try to drop and catch the postcard
yourself you will find you are successful at it, because your brain
can send "let it go" and "catch" signals to your two hands at the same
But when you hold the postcard for your friend to catch, his/her brain
must first see that the postcard is falling, then send a "catch"
signal to his fingers. This takes more time. That is why your friend
can't catch it.