Receivers in leagues that allow forward motion can start the play racing down the field, perhaps allowing them to go past defenders. Only players in the backfield and not on the line of scrimmage may be in motion at the time of the snap in all forms of football. In American football, forward motion is when a player who is not in the backfield and is therefore considered to be a receiver starts running toward the end zone or another player who is not in the backfield and is therefore also considered to be a receiver begins running after the ball is snapped. The term is used because it appears as though the players are moving forward even though they may actually be going sideways or backwards. In Australian rules football, forward movement is when a player has moved at least five metres from their original position before the ball is kicked. This could be either forwards or backwards, depending on where the player was positioned prior to the kick. In rugby league, forward movement is when a player has moved more than ten yards from their previous position without returning to midfield. A player can also be awarded a penalty if they enter the opposition's half of the field when offside. In rugby union, players are allowed to move up to seven yards horizontally and vertically without being declared out of bounds.