Not sure if "Underhand" is the correct terminology; whatever the following cast is I find it very useful while fishing tight to shorelines w/ lots of overhanging brush, branches, degree, etc...
Technique: Back cast is a normal overhand back cast, maybe cheating a little bit to sidearm. Start a gentle forward cast with your casting arm fully extended and high. A normal forward cast would have your casting hand following an approximately parallel path to the waterline. During the underhand cast you want to dip your hand below the parallel plan as you move forward reaching the maximum "dip" as your hand passes your body. Then follow thru by raising hand/arm high, back to atleast the original height of your hand at the beginning of the forward cast.
The dip causes the loop to open up underneath the main fly line, and the fly on the end of the line will almost hit the water and then travel upwards for the last couple feet of carry. As you perfect the amount of "dip" and follow-thru you will learn to control how much of an undercut you flip the fly under.
Early in your learning you will definitely over do it and flip your fly up into the branches, but w/ practice you will find this technique is amazing at getting flies into tight quarters that don't let you use a full sidearm cast.
I do have an illustration of this technique that I drew up that I will gladly share.
I once knew a drug addict... He told me I have a fishing problem!