To bend is to curve. You can bend physically, as when you practice yoga, and also figuratively, the way you do when you "bend over backwards," working hard to make something happen, especially on behalf of another person.
Tree trunks and branches are able to bend when they're whipped by strong wind — this way, they're usually able to avoid breaking. A bend in the road is a curve, and to bend the rules is to change them slightly in a specific instance. If you try to do a backbend, you have to arch or bend your spine and lean back until her head hangs down and your hands touch the floor. It's a move for the young and flexible. The Old English root, bendan, specifically referred to bending or stretching a bow.