SNIP

I suppose it relates to how flat the polar curve is
above 100 kph

SNIP

Okay, I'll bite. If glider penetration describes the
ability of a ship to make progress into the wind without
falling out of the sky, is it soley a matter of looking
at the polar at the indicated airspeed? Intuition
tells me that is it, but an aeronautical engineer type
might make be look like a big dummy here.