Generally, if you complete the half twist before you finish the first half flip (ie before you're upside-down), it will be recognized as an arabian. Any later, and it's a 1/2.
in an arabian, you turn from back to front before the flip is started- then its a forward flip.
I think you are talking about a back flip w/ a 1/2 twist? If so, the flip is initiated backwards, then the twist happens as you get your feet closer to the floor.
They are 2 totatlaly different skills w/ different techniques for takeoff, turning & flipping.