The key is when the ball is snapped. You can one person moving (motion) when the ball is snapped. For example , the jet motions they use. And can't go toward the line of scrimmage.

Prior to the snap you could have multiple people moving simultaneously. This is a shift. With a shift all players on the offense must be still for one second prior to the snap.

Henderson is performing a shift. Notice how the snap is delayed and he is still once he lines up in the a gap. Once he has been still for a second, you can start new motions.

Hope that helps.