The thing about a Pratt muffler is this. It doesn't strap over the bearing edge. It comes up against the head from inside with progressive tension. So; if you strike the drum for an open note, it stays open. If you strike the drum for a closed note it stays closed because you are then causing the head and muffler to then connect and the muffler becomes a factor all of a sudden. You can be playing with resounding open tones one minute and closed punchy ones the next or a combination if you carefully adjust the muffler relative to the beater strength.
Calfskin,
If I follow you correctly you are saying that when one uses an internal tension muffler like a Pratt muffler, by depressing the head for a strike and then leaving the beater depressed, you are effectively pressing against the muffler thus keeping the head muffled. If you strike it open the head would still vibrate against the muffler but not to the degree of being held there by the pedal beater thus creating a more open tone.
Now that makes sense to me. I am not sure I like the idea, but I believe I follow it and sure would like to try a drum or two or three that had one installed.
By the way, if anyone is looking for a more modern recorded version of that kick sound, check out "New Mistake" on the Jellyfish album Spilt Milk. Monster kick drum sound and a very interesting band. They are what you would get if you blended the Beatles, Queen, Led Zepplin, and possibly a hint of ELO with the sense of humor of Frank Zappa. All the while the drummer is singing lead while playing standing up. Worth a listen.