I'm not quite sure that counts as practice - sounds more like something you'd practice FOR :biggrin1:
Why can't it be both? If you have something large inside you to flex your pussy around, as opposed squeezing with an empty vagina, surely the workout would be the same (if not better).
The added bonus is the instant feedback too. If you want to isolate the muscles around your deep spot, you could start off randomly firing off different muscle groups (ie push out like you're trying to force your pee out, as opposed to flex you anal sphincter, as opposed to try sucking the bottom of your stomach up). If I had my cock inside you, I could say if you were squeezing down on my glans, as opposed to flexing around my shaft, or squeezing around my base.
Of course we'd take breaks for orgasms, but then get back to the grind after you've floated back down.
The "workout" also goes both ways. In PE there's a routine called "erect bends". Rather than using both hands to bend my cock, I could use you're entrance as a fulcrum, and raise my hips above (or below) the entrance. Sort of like a crowbar effect of sorts. Sometimes I can inadvertently put an enormous amount of pressure on the g-spot (depending on how shallow or deep my cock is). <-- that pronebone position is very usefuil for this one.