For the Player to 

fall through the platform from above

 we need custom code:

This code works by:

Checking to see if the player is pressing the down key

If they ARE we stop looking for collisions with the pass_through layer (Mask Layer 3)

If they ARE NOT we re-enable collisions with the pass_through layer (Mask 3)

Call this new function in process,

then check that it works as expected.