"There is also an out compression which is rarely applied but is used in kundalini yoga. This is with the abdomen being pushed outwards and held firm, while the yogi is vigilant to perceive energy movements with the psyche which may result."

My experience with the outward abdomen compression:

I use the abdomen out compression lock on occasions. I find it interesting in the sense that when there has been sufficient admixture of sex hormonal substances and infused breath, it can be as efficient in propelling the life force out of its basal posture as the regular compression.

I normally apply it only in standing position. Even though the abdomen is pocked outward, my instinct applying it which started about 3 years ago, is still aware that there is compression all around. Although they are directed differently, the diaphragm, the rib cage, and the abdomen muscles are still involved, even if coordinated differently.

I feel that the lower abdomen, in the reverse abdomen compression is not pulled in as tightly as in the diagram above, though there is pressure applied inward, which loops into the small of the back more so than otherwise. And of course, the psychological locks remain equally important.