Well, once I sparred with a TKD guy who was always with a leg up ready for side kick, or other. Yes, it was effective. I had a hard time figuring out who I could reach him. And, generally speaking, TKD guys do it often in competition. So it works, in some contexts.In popular movies and video games, I've sometimes seen a person kick their target multiple times without putting their kicking foot back on the ground. I have a hard time believing that this could be effective because it seems like there's hardly any space to create power or momentum for the kick. Am I wrong? With enough training could this be effective?
From my side, I use two kicks combos, being the first only a feint / touch kick (or turning a real kick in the last moment if no defence). For me is dificult more than one power kick, but that one can be effective (such as the 'question mark kick' mentioned here).