The same inside/outside division shows up in some old European texts, if I remember right. Basically, the front of your body (guts, groin, chest) is on the inside, and your back and butt are on the outside. Since with a spear, you are standing with one foot forward, this divides each side into left and right. I think the "circle" is meant metaphorically, rather than literally.
From reading bits of the English translation (
http://www.chineselongsword.com/speartranslation.shtml), I'd say that the general idea is that inside/outside are based on which foot is forward, which varies depending on which stance you are in and where the front of your body is facing. It appears to assume that your opponent is standing with right foot forward, but that's just in the free samples they provide.
I'd take their spear descriptions and illustrations with a small boulder of salt. The spears described are the equivalent of European pikes, but the pictures show a spear about two meters long. They're somewhat different weapons, so be careful.
Another point is that the video with the translation should be watched carefully. In particular, pay attention to how often the demonstrator locks his elbow (bad idea), where the blade is (there's one point where he chops with the flat of the blade). Also note that his feet are reversed in the "end stance" (in bagua, you steal a step by bringing your back foot behind your front foot, because you can do so without moving your upper body. You can then lunge forward with some faint possibility of surprise), and ask whether he can generate any power with that bizarre back leg with the toe pointed back (the illustration could equally show someone with front toes forward, back toes out 90 degrees, or the classic fencing stance). The "jolt the legs" phrase is probably best interpreted as using your legs and back to power the spear, rather than using only your arm muscles. Spears are heavy, and legs are strong.
My personal favorite introduction to the spear is from Cold Steel (
www.coldsteel.com/files/Riposte/The-Spear.pdf). It demystifies spears in a hurry, and it has the bonus of being free.
My 0.0000002 cents,
F