I call this defensive position the “sleeping position” because it resembles my arm positioning when I sleep. It’s quite comfortable, and I have yet to experience shoulder pressure while I’m sleeping, so double success!
This arm positioning serves a few purposes. First, the bottom arm prevents your opponent from connecting to you to apply shoulder pressure to your neck. The top arm, specifically the elbow position being slightly in front of the ribs rather than on the side, both prevents the top player from getting the underhook while facilitating your own underhook. It also keeps your top shoulder rolled forward which keeps your structure solid and able to support weight. Lastly, it keeps your arms under your opponent’s torso in a way which help some of the off-balancing we will do later on.