As you saw from the last screen, the reveal drawing method uses an alphaMania castmember to 'cut through' to a bitmap castmember. That bitmap castmember is known as the sourceMember
In addition to the sourceMember, there is a flag, the sourceRelToSprite, this governs whether the sourceMember is displayed as if it where 'on the stage', or if it affixed to the sprite. Drag around the heart mask above and play with the buttons on the right.
