You hang the underquilt under your hammock. If the lines that you are hanging the underquilt with squish the underquilt too tightly against the bottom of the hammock (top of the underquilt), you will get compression. It's a little tricky, because you want to hang it tight enough against the hammock to not get large air gaps, and at the same time you don't want the underquilt plastered to the bottom of your hammock (compressed). There's a fine line there. The cold hip in question points to an underquilt possibly getting compressed at that particular point. The answer given was to see if the underquilt could be hung a tad more loosely so that it won't compress at the hip.
Hope this helps, I'm sure it's clear as mud.