You only have a rate of descent when descending
... seems obvious but
ain't necessarily so when talking quickstops and vortex ring...
..... the problem of "rate of descent" when approaching vortex ring is in fact flow upwards through the disc opposing induced flow (hence "power on" being a prerequisite for vortex ring)...
... in a level flare with forward motion (aka a quickstop) you have precisely those conditions. And of course for all the downwind hover and rearward airspeed reasons you would have much more power applied if you tried to quickstop downwind.
Not sure why anyone would
want to do a quickstop downwind, when a "flare and turn" of some sort can almost always be accomplished.
Did have a stude once try and quickstop (a heavy Wessex) downwind when invited to "do a downwind quickstop of your choice"....
... he was subsequently chopped! But not just for that...