Since you only slide a rail for like a second it is a good idea to practise your balance first. One good way of doing this is by using a skateboard as a tool. You can also try just standing on one foot like I show or stand on a balance board.

You will have to move your center of gravity over you back foot. I like to do this by showing the hip to the side and slightly bending the body.

I like to have to front foot a bit high and with the hip moved backwards when I slide long rails or when I want to do backslide to backside 270 out. When doing some other spin variations it is not ideal.

When doing tricks like 270 on to backslide or k-fed backslide it is better to pull the foot straight up since this way you will spin faster.

Try balancing just standing still on a skateboard like this. Play with the hips from side to side to maintain balance until you get a nice feeling for how to balance on a one foot on a skateboard.

Start rolling on the flat on one foot. Then find a easy downhill street where you can practise you balance for a "long" time.

Pay attention to the shape of my body. The hip is moved back so I am nicely balanced over my back foot.

Rolling down a street like this is more realistic than rolling on a flat street since you have to lean a bit forwards just like you would have to on a down box or rail.

Notice that I am leaning a bit forwards/downhill to stay balanced on this down rail.

To make it harder find a short, low flat box or fat tube.

First times you try this you definitely rather want to lean a bit to much forwards than backwards.

When you lean to much forwards you can avoid crashing by just putting down the front foot on the box or rail.

Approach the box or rail you are trying this on with a gentle wind up and set of the rotation so you spin fast enough to rotate the 90° on.

Rotate 90° on to the box and also make sure you land with the ski completely flat against the box.

Try to to not rotate on the box so you are on the nice 90° angle even at the end of the box. Having you hands a bit wide helps slowing down the rotation.

If you have to, use a bit of counterrotation to adjust your rotation so you land nicely backwards. If Gary here did not counterrotate he would have landed crooked which isn´t smooth.

I often have my hands high and wide for good balance and it also helps you to slow down the rotation while you are sliding then when I jump of I pull in the arms to speed up the rotation again so I land switch in a nice way.

Having the arms high and wide can look pretty ugly but it is so practical since it help me to be able to to backslide 270s out with ease.

To backslide a whole box or rail you must in general lean back more than you think.

IMPORTANT: When you jump onto the rail it is important that you pop perfectly high and pull up your legs a bit. Perfect is when you jump on and never drop on to it you just land on it with little or no drop on to it.

Then you push down the back foot onto the rail. This is very important in order to maintain good balance and control. If you jump too high and you drop onto you back foot you will loose some balance sand control.

Try to land on the back foot with the leg slightly bent.

Landing on the edge of the box like this is not cool!

Rotating to slow, also not cool!

Set of just enough rotation to do a perfect 90° rotation onto the box or rail!

Land on the box at 90° spin with the ski completely flat on top of the box.

You will also have to lean a bit forwards so you are perpendicular to the box and you will have a great time learning this trick!

The chicken foot is a way more common problem than crashing on your hip. The chicken foot comes down when you where leaning to much forwards which led to putting down the foot and not completing the backslide. This can be very frustrating but almost never painful. Lean a tiny bit more back/uphill next time!

You wand to find something in between this crash and the chicken foot. If this happens lean more forwards. With lots of practise you will find the sweet spot where you get a nice balance.

Practise many many times and you will learn how to find balance on a steep down rail know how to change it for a flat rail or up rail.