The front side 180 is a common air on a skateboard while riding a ramp.
This trick is performed by using the law of conservation of angular momentum.
This law states that if you're rotating, you'll keep on rotating unless a twisting force stops you.
Also, in the case of the front side 180, if you're not rotating, you need a twisting force, or torque, to help you to start rotating.
So you use your arms as torque to turn yourself around in mid-air.
The law of conservation of angular momentum is one of the forces at play when turning in mid air on skateboard
If you where to jump and rotate your legs 90 degrees beneath you,
your arms and torso rotate in the opposite direction.
If you stick your arms out as you turned
this increases the rotational inertia of your upper body.
What happens is that a large rotation of your legs is exactly cancelled by a small rotation of your outspread arms.
Since the two rotations cancel, angular momentum stays constant at zero, and the law of conservation of angular momentum is satisfied.
Skateboarders turn in midair by twisting their arms and legs in opposite directions.
Upon landing, a skater can use the friction between his/her feet and the skateboard to twist the upper body back into alignment.