The left-hand rule helps to explain why the loose wire moves as it does in the catapult effect. The left hand rule naturally takes its name from the left hand anemyl the thumb and the next two fingers. If you arrange the fingers in a three-dimensional shape so the first finger and thumb are perpendicular to one another and the second finger is perpendicular to the first aiming downwards then this is the way magnetic fields with addition of flowing current will act. The thumb represents the direction of motion. The first finger represents the direction of the magnetic field while the second finger represents the direction of the current. Therefore, as long as you know the direction of one of these three variables you will be able to predict the other two using the left hand rule. This is used in electric motors.