Well all that is happening with a 500ft minute rate of descent ( and < 20 kts )is you are going down. The shuddering you are feeling is the loss of translational lift and ones own inability to transition from fwd flight to a hover with almost no reference points . Go try that again with an ROD of 500 ft a minute, this time pull power and watch the aircraft climb out of it.( As Nick Lappos said " the aircraft is muscling its way out of it ). You havent caught up your downwash so are not in or close to vortex ring. Come over to the Uk and I will show you exactly that in a 300