Parowan Fatal Crash

On Jul 1, 2:32*pm, bildan wrote:
Any attempt to turn will trip an asymmetric stall which will develop
into a spin in less than a second. * *Just before the glider spins,
everything "looks" normal. *(Except, of course, for the ridiculously
low airspeed, sloppy controls, absence of wind noise etc...) *Several
high time glider pilots doing a BFR with me didn't see this one coming
and were visibly shaken by their 'inadvertent spin'.

What happens next is crucial. *If the pilot does nothing, the glider
is likely to transition into a spiral dive. *
I do not understand this last sentence. What converts a spin into a
spiral dive? What is the aerdynamic/physics here?
Heinz