It goes about the red player.
When the red player wants in his turn to move 5 passes (speed) and 1 drift, where does he end?
He first moves 4 passes straight ahead AND wants to drift 1.
Will he be then 2 spots before green lande or 3 spots ahead of green lane?
Due to the nature of the curve spaces Red would end their move two spaces ahead of Green. This is because the front edge of that space extends beyond space 4 so it is considered forward movement. To get to the space three spaces in front of Green Red would need to move one more space straight ahead before drifting.
Edit: If the Red player went to space 2 or 3, then drifted and then went straight they would end their move 3 spaces ahead of Green.