The casper disaster is possibly the first trick ever given the name of “casper”, after its inventor, Bobby “Casper” Boyden. Although we don’t think of it as a casper now, you can see how it relates; one foot is on “top” of the board while it’s in rail, and the other foot stays on the griptape, pushing it through a 180º spin.
With a bit of practice, you can do a lot with this trick; it flows quite naturally into all sorts of footwork and can be spun into much higher rotations – making it a very worthy trick on its own.

You want to set up for a casper disaster like you’re going into Heelside Rail; the only major difference is that you want your front foot at the front truck for more control once you’re spinning.

Roll backwards at a low speed, and keep your shoulders facing the nose. This is important – if your shoulders are in line with the board, you won’t be able to generate any rotation.

Before I even begin to lift the board to rail, I need to prewind by twisting my shoulders in the opposite direction to the spin. This is where the rotation comes from.

I start compressing at the knees as I unwind, ready to create the moment of “weightlessness” I need to lift the board into rail.

As my shoulders come into line with the board, I start lifting the board to rail in exactly the same manner as I would with a regular Heelside Rail – the only difference being I’m not going to lift my front foot.

While my back foot pulls the board up onto the rail, my shoulders keep turning. They want to be ahead of the board all the way through this trick, or I’m going to stop before I get to 180º.

This is the classic casper disaster position. My weight is almost all on the ball of my back foot, allowing the front end of the board to raise up slightly. My body is still twisting.

As I get to 90º, you can see what the front foot is doing to bring the board round – I’m using the side of my foot to push on the griptape as my front leg follows my shoulders. It’s important to use the side of the foot; the toes create too small a contact area, and might flip the board.

The trick is almost done at this point. My shoulders can stop rotating now – my board and shoulders should end up in the same orientation if I want to roll away cleanly at 180º.

To get down from the casper disaster, you push slightly on the back foot in much the same way as you’d drop down from rail normally. Don’t let the front foot do it or the board will spin away from you.

One of the joys of this trick is that the front foot never leaves the griptape. As I come down, I’m still touching the grip with the front foot, and I have full control.

Nowadays I find it quite hard to simply stop once all four wheels are down; normally I keep that rotational momentum going with some spins or footwork. Once you’ve mastered this one, play around and see what you can follow it with.
Additional Notes
While you can learn this without freestyle wheels, it’s not recommended; the sound of axle nuts on asphalt is horrible, and it’ll ruin your trucks. You’ll also run a high risk of locking up mid-rotation as your axle gets stuck in a crack. If this happens and you’re going fast enough, it’ll be a slam you’ll struggle to get away from.
If you start playing around with larger rotations, remember that the general rule of thumb with these is you always want to roll away forwards. A 180º rolling forwards will stop you dead; similarly, a 360º casper disaster rolling backwards will feel horrible.
That said, this trick tip is based on the assumption that you learned to go to rail using the back foot hooked over the back wheel. If you prefer to hook your front foot, you’re going to want to learn these rolling forwards, and will always want to roll away backwards (or land to an endover on the back wheels) to preserve your momentum.
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