It’s not directly related to either the psoas or the pelvic floor muscles (though it’s not far removed from either), but warrior 2 is a logical next place to go with the chair series. Physically, it’s not too far removed from the chair options for warrior 1:
[Video from Expert Village via YouTube.]
For me, warrior 2 has always been a huge strength pose. At various times, I’ve felt the work concentrated in my front quad, my front glute, my rear glute, my rear calf and hamstring, my pelvic floor, and my abs. Because it’s such an active, energetic pose, it’s a lot of work for me to send my body out in space like that. When I want to focus on alignment — to make sure I’m working the right muscles in the right way without hurting other parts of my body — a chair is helpful for figuring out which parts go where. When I’m comfortable with the pose in-chair, I’ll usually experiment by lifting myself up about an inch or so — enough that I’m supporting my own weight, but low enough so that if my body responds unexpectedly, the chair is close enough to fall into without hurting my butt.
Also, not gonna lie, sometimes, if I am sitting at work or something (which doesn’t happen often), it can be an energizing stretch break that no one knows I did. I like to call these Secret Desk Warriors. 😉
While I’ve never used a chair as a standing prop for this asana, I remember the burning in various parts of my legs, and I can certainly imagine where it would be useful:
[Video by Expert Village via YouTube.]
In the way I’m picturing it, at least, this version would involve the legs supporting most of the body weight, with the option to rest some of the upper body weight on the chair. Additionally, depending on individual needs, the amount of weight resting in the chair could be gradually reduced — in other words, one might be using the chair for a lot of weight support, for a little weight support, or mostly for balance (or anywhere in between two of those points).
Ultimately, I’m very much a fan of modifying strength poses — like the various warrior poses — with chair options. They end up more accessible, not only to a wider variety of people, but also in a wider variety of needs and circumstances.