The low  straight -push - thrust and roundhouse kicks are exceptions to this rule. There are many flying kicks in Muay Thai, however, they are not used as often as the other kicks because such maneuvers places the element of risk to the Muay Thai fighter in terms of balance especially when one's opponent is on his or her best guard. 

It is preferable to make good use of the low push - straight and thrust kicks  and low round kicks because they do not sacrifice too much balance nor energy,  and, repeated attacks using the low round kicks to the opponent's legs can lead to the inability of the opponent to stand or fight using his or her legs.

  There is an elevating maneuver in which one leg is used to step upon one of the opponent's thighs acting as the elevating support while the other leg  executes a round kick. Sometimes, one hand is used for grabbing the opponent's neck or shoulder so that balance is maintained during execution.