The bottom of the skirt DOES close the exhaust port when the piston is at TDC.

But, I think whats being said, is that if you LOWERthe exhaust port too much--you risk the skirt actually OPENING the exhaust port at the bottom when the piston is at TDC. You've just opened the exhaust port with the bottom of the skirt--and exposed the crankcase charge under the piston to the exhaust.

Come on Ralph--keep up.