MoveBackward Func

Good Evening,

Im playing a bit with GWA2 trying to understand the limits and doing random stuff.
Im trying to moving backwards but im not understanding the variable i have to put into it.

Func MoveBackward($aMove)
If $aMove Then
Return PerformAction(0xAC, 0x1E)
Else
Return PerformAction(0xAC, 0x20)
EndIf
EndFunc

What is $aMove?

Sorry and thank you if its a stupid question

$aMove is 0/1 False/True

on True it will act as if you held your down arrow key down, and called again on False will act as if you release the key