The statement that is not true about a soccer throw-in is:
Both feet must stay in contact with the ground.
During a throw-in, a player must have both feet on or behind the touchline, but they do not have to remain in contact with the ground; they can lift one foot as long as they fulfill the other requirements for a legal throw-in. The key requirements are that both hands must be on the ball, the ball must come from behind the head, and the ball can indeed be thrown with sidespin as long as the throw-in is executed properly.