What is Vertical Axis Spin, and how does it happen?
After your takeoff, as you rise through the air, you will be spinning as well. The most noticeable spin is about an axis parallel to the bar. But, there is another more subtle spin taking place at the same time; a spin about the vertical axis.
The spin about the vertical axis is the spin that will turn your back toward the bar as you rise off your takeoff foot. The best way to start this spin is to drive the knee of your lead leg slightly across your body as you drive it up during the takeoff stroke. At the moment you leave the ground, the thigh of your lead leg should be parallel to the ground and at a slight angle away from the plane of the bar. This small movement of your lead knee will provide ample spin about the vertical axis.
This might seem odd at first, because it takes a comparatively large effort to get enough spin about the axis parallel to the bar. In both cases, you will be spinning approximately 180 degrees. The difference stems from the fact that, for a large portion of your flight, your body is very compact about the vertical axis. (This is in contrast with the spin about the bar, where your body is laid out more or less lengthwise.) As you leave the ground, you are very much like an ice skater in a tight vertical spin. As you reach the high point of your flight you are laying on your back so you spin slowly like an ice skater with arms and legs extended. And, as you descend toward the pit, your body is once again oriented vertically and the spin rate increases.
There is no need to twist your shoulders around to get your back toward the bar, or to perform any other of the gyrations that jumpers often do. In fact, such gyrations and twisting absorb much energy from your jump, Whereas a little cross body drive from your knee costs very little and provides plenty of vertical axis spin.