What it strengthens:
Obliques, abs, back, glutes
This exercise feels so good for all the tight muscles in the torso and arms. Not only does it strengthen nearly everything in and around your torso, but it gives them all a great stretch too!
Stand with feet together. Lift both arms above your head and press your palms together (interlock your hands if desired), keeping your arms next to your ears. Breathe out and lean your arms to one side, pressing your hips out in the opposite direction. Do not lean forward or backward but move to the side, as though squashed between two panes of glass. Keep your abs pulled in and butt and legs squeezed tight. When you breathe in, straighten back up and on the next out breath, lean to the other side.
To make easier:
Use only one arm to reach overhead and use the other to support yourself on your hip or on a chair/counter top (more like the modified windmill) and/or spread your feet wider for more support.
To make harder:
Bend further and/or hold the bend for a few breaths before straightening.