I've got to admit, I've never learned this trick. Haha get it? No? Okay...
Anyway, I've been wanting to make some hats/masks with a semi-transparent area with colour where a player's eyes are placed. Like on the PK hats and gas masks kinda. Here's an example of what I mean, and don't worry I pulled this off Google:
Uhh the image is tooooooooooo big, so I'm putting it in a spoiler.
Spoiler
If anyone knows how to do this, it would be very much appreciated. ^_^