creating gradient mask from shape layer with gradient...

I've seen it done, but I can't find the tutorial. I have a pic that has a hard edge on the bottom, I wanna soften that edge in a way that I can turn the effect off later if needed. I've seen it where I create a rectangle over the bottom of the pic and in a new layer, apply a black to white gradient (using a layer style so I can adjust the gradient), but I can not figure out what to do from there to make it mask off the image below it in the layers panel - can someone help?


Thanks all.

Make sure to check out my Heroes Fan Film at http://ruesterprod.blip.tv