Flop usually means a process where something naturally falls or slumps to a position. Some front wheels seem to want to assume a certain angle when the bike is parked and sitting on a stand. Would that be it?
In days of old, long ago when the world was flat, you could ride a bike no hands. It would track straight and you could steer by leaning into corners. I have tried this carefully with a number of used bikes of all types in recent times and I haven't found one that would allow you to do that or, not to the point where I was prepared to risk it. Maybe where I rode years ago did not have significant surface camber. Has bike design changed? Is it a result of flop or trail?
Push it along by the saddle [ recumbent's seat back ] can it go in a straight line ?
If no then what happens to it ?
It turns either way and the turn tightens as it moves forward , by just holding the saddle you cannot get the bike to ' unturn ' and straighten up = wheel flop [ try hands free , loose some teeth ]
If turns either way you can get it back on a straight course by just leaning it the opposite way = mild flop easily countered [ this bike would ride easily hands free ]