Scott was hit by the darts, slager more then likely got the wires wrapped around his wrist when Scott got up and fought for control of the taser.
That's what I saw when I looked at the new short video at the Treehouse. There appears to be a dart in Walter Scott's leg, and the wire wraps around Michael Slager's arm as he raises it to shoot.
We know the wire is wrapped around Slager's arm, because the cartridge doesn't move at all when Scott is running.