Looking at an underused effect in After Effects, Eran Stern demonstrates a great use for Extract using the effect in place of a color key to preserve more details than a key would under certain situations.

This effect creates transparency by keying out a specified brightness range, based on a histogram of a specified channel

Eran provides an example of integrating titles with some billowing smoke, using the Extract effect in After Effects to create transparency in an image based on its luminance or best contrast color channels. Check out the tutorial looking at the power of the After Effects Extract Effect here.