Use F4 to hide or reveal the 'TrkMat' column.
When you choose 'Alpha Matte' from the drop down menu, After Effects will use the layer above as a 'matte'. This will mask off the bottom layer, using the top layer's Alpha Channel as a 'cookie cutter'.
By default, 'matte' layers become invisible, but the eyeball can be manually turned back on if necessary (usually to temporarily view the matte itself).
I personally always color code 'matte' layers RED. The layers that 'use track mattes' I color code YELLOW. This way, at a quick glance I can tell everything is correct when RED layers are on top of YELLOW ones.
Thanks to Demian Dunkley for inspiring me to make this video.
more at FXDOJO.COM
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