From their website
EVAC HARNESS
EMERGCO’s evacuation harness (“screamer suit”) is a helicopter rescue personnel-carrying device system (PCDS) that is easy and quick to put on. It is designed to physically encompass the torso to prevent the person from inadvertently falling out of the PCDS, and is safe to use with disabled or unconscious persons.
The harness is put on like a jacket—arms go through the armholes and a bottom flap comes up between the legs to a three-point connection at an approved snap link for attachment to a winch cable hook or other external load suspension line system master attachment point (MAP). The Evac Harness is ideal for use as part of Emergco’s Helicopter External Transport System (HETS).
The Deluxe Evac Harness is similar to Emergco’s Evac Harness, with the additional benefits of grips along the top edge, flotation band sewn into the collar, and a hole in the seat to drain water, making it more suited for use in marine rescue.
difficult to see how anyone could fall out of that.
and how would that stop her from falling?
falling from 20 feet is much more likely to be survivable - yes we don't know their performance calcs or OEI capability but that still seems very high. A lateral move to better ground to ensure she is fully locked into the harness before the long winch in might have been prudent.