The underhand cast is a variation of the sidecast, the difference being that with this variation the caster throws the loop underneath the tip of the rod. This is achieved by casting the side stroke with a small downwards dip with the tip of the rod ( i.e. the tip of the rod draws a saucer shape during the casting stroke).

The result should be an upsidedown loop. The smaller the dip, the tighter the loop. This variation is particularly useful for casting beneath trees and is often the only workable tactic - even if the loop hits the overhanging branch, the fly often kicks underneath and still lands on target! A very useful cast on small steams.

One advantage of the underhand loop is that with a strong positive stop after the loop has straightened the end of the flyline kicks upwards. Yet another good reason to learn this neat trick!

An advanced technique is to make an underhand cast on the back cast, which forces the loop to pop upwards (great if you have thistles behind you, or just a big heavy fly), followed with an overhead delivery on the forward cast.

Because this is flycasting...
Note well: the title 'Underhand Cast' is also currently being used to describe the Single Spey cast. The main emphasis being that the lower hand on a double-handed rod doing the work. Historically the Underhand Cast was also used to describe the roll cast. Many USA instructors refer to this cast as the Pendulum Cast.