Thanks, Carson.
A Semi-sumo stance is a very narrow sumo stance - legs just outside arms. I've also seen it referred to as a "modified" or "modified sumo" stance. In a conventional stance, I can achieve and maintain thoracic extension only without shoes, and only with a DOH grip. Switching to a hook or mixed grip (or wearing lifting shoes) requires me to bend over just a hair more, and I can no longer get into good extension. I found that my back angle is not drastically different with a semi-sumo stance than in a conventional stance, but it allows me to get into good extension and maintain it. I'm sure that some will think that I just need to learn to get into, and maintain extension in a conventional stance, but that's perhaps a discussion for another time. I'm more interested in getting stronger than following the SS model to the letter - although I certainly value the model and the reasoning behind it.