What they said.

Also, I live in a place where we get multiple warm nights, and I'm a particularly warm sleeper. When it's too warm for the underquilt (even vented), I simply push it to one side of the hammock. If it gets cold enough during the night, I reach under the hammock and drag it back into place.

Same as you would with a top quilt, except that the underquilt is, well, underneath you...