You're thinking about it from the ass end back...

When you do a guard sweep, you're getting into a guard position (give me a second, I'll explain what that is) and basically doing a turnover from there. You don't need a really high, extravagant throw arc. All that matters is that you pull guard one way or the other, put your hand on the ground, and pick them up and put them on their back.

Now to what a guard is: In Toribash we don't really distinguish between guarding and mounting as real martial arts do, so take the terms "guarding" and "mounting" as synonyms.

I'll upload a few full guard replays and the guard sweep I used to make it in the tier.

Notice that in all the replays I wrap my legs around my opponent's torso and use it to manipulate them. That's what guarding/mounting is.