The Kimura is also called the "reverse paintbrush" because it is explained that you put their hand on their spine and paint a line up to their head. The side-effect of that explanation is that you are compressing the arm at the same time. As jnp said, that detail (compressing) isn't mentioned much. I had to figure it out myself. :-(
Originally Posted by Vince Tortelli
Of course, if something is working for you, keep trying it. If it eventually stops working or you determine it is low percentage, reverse engineer it, reassess it, and reapply it in a different way. I'm constantly doing that.
My Kimura from guard has a slight variation that gives me a lot of leverage: after their posture is broken down and the Kimura is locked and cinched (take away all space, elbows to my body), I shift my hips up under their shoulder to give myself maximum leverage to pull the hand into the armpit when my leg goes over their back. If their hand goes behind their back a little, no problem. The key is the angle of compression, not the placement of the hand.
Different people have different builds so setting up your hips to provide proper leverage is different for everybody. I just focus on getting my hips close to their shoulder and the angle/distance/pressure seems to work out.