MY 2 cents:
I go knee on chest when I am ready to work through baseball choke, loop choke/collar choke, armbar series. Otherwise I stay knee on belly because I feel it opens up more scramble oportunities (that I win because I driving my oppenent throught the mat).
Thanks for the article.
My Sambo coach was really big on the idea that one of the main purposes of a pin was to compress the diaphragm and inhibit breathing, and thus was a big advocate of the knee-on-solar-plexus variant.
Personally, I will only do this when rolling with bigger guys or in a competitive setting; crushing a smaller fellows lungs with a pin in rolling just seems unnecessarily cruel and unlikely to help me develop better game.
I normally use a knee-on-chest position when I am setting up a far side armbar. It just gets me in better alignment for the transition, rather than ending up way out of position. I am a bigger guy, so I prefer to not use thisin class, but I will use it with gusto in competition.
It's a great tool to have. My game is KOB heavy, But I like to switch it up. Different levels of pressure (hard-none), different angles, different heights... I don't play a lot of heavy KOB unless I'm tired. In general I try to move as much as possible and catch armbars or back takes in transition.