Any misalignment of C-1 (Atlas) can cause inflammation that can interfere with nerve function, blood flow to the brain and blood flow to the brain stem. The idea of C-1 compressing the brain stem is an over-simplification. The compression is from the inflammation, not the position of C-1.

There may also be pinching of nerve fibers but there are are not nerve fibers in or very near the joint areas (occipital condyle to Atlas) like there are near the highly moveable joints of the lower vertebra.

That is why icing the area is so valuable. Anything to reduce the inflammation is beneficial.

The blood flow being restricted is likely the cause of the PCS symptoms since they manifest from areas that are not necessarily near the C-1 structures.

An important posture issue to be aware of is moving the head laterally (rotating side to side) at the same time as moving the head up and down. In other words, don't combine Yes movements with No movements. These combined movements are most likely to cause impingement in the joint increasing inflammation.