While it's a nice (misleading) cue wrt swinging when you want to vs swinging when you have to after getting your foot down...the front foot has to be down to leverage the swing against for full efficiency.
Sure, a club or bat can be swung with the front foot in the air (except if you're told to "swing down" to it)...
...just not as well, as powerfully, or as efficiently as when it's planted firmly on the ground.
exactly! Or your told to get on plane early or behind it. Which makes the axis point change etc etc.