If you're hit when you light shield you're pushed back farther than if you used the full shield.
Hitting someone in their shield when they're near the edge of a platform (or the stage) will push them off. In the right situation, it can knock someone into holding the edge.
The idea here is Marth's Up-B will actually knock you back down to the edge to edge hog. You have to be holding the stick away from the stage, so you fall back instead of back onto the stage. Additionally, you must be facing the stage (can't grab the edge if you're facing the wrong way). In order for this to work you have to fall fast enough to get the edge after getting hit with the up-b but before Marth does. The window isn't huge, and makes this harder with floatier characters. Usually you'll see this done with fast fallers like fox or Capt'n. I believe it is possible to manually fastfall in that window to slightly lessen the timeframe Marth has to grab the edge, but I'm not sure.
I don't think IC's fall fast enough, but I haven't tried this with them. It should be relatively easy to test with a friend. First do it with a fastfaller that you know it will work with to make sure you get the idea, then try with the IC's.
I've found while this technique is cool, manually wavedashing off to edgehog works fine. IC's have an awesome wavedash, I'd most likely go about it that way (although my IC's aren't the best, someone else here may have better advice).