Hi David, they have a 'stop screw' you set and then you can lower a string to a given pitch, a'la like a steel guitar sound. When you release the palm pedal, the string returns to its original pitch (if you're lucky) - -therein lies the problem, sometimes it may 'drag' a little and not come all the way back to pitch - - and THAT will drive you nuts
I never really thought of it as a "Gretsch gimmick", it was just another accessory you could add on any guitar, At that time late 60's, early 70's they were fairly popular around the Nashville circles,,,,guitar players trying to get a steel sound. Phil Baugh had a complete pedal setup on his Peavey 6 string,,,,floor pedals and all. Shot Jackson made that for him down at ShoBud. He could pull all six strings with his foot as he played. He was good at it and worked a lot of TV shows back then.
Gayle