We call this the chimp because we start of bending sideways as if we are chimps wanting to scratch our armpits! When we reach the armpit, we take the one arm over as shown in the photo. This we then hold for a few seconds.
I use this at the beginning of my yoga class to warm them up. It isn't strenous and yet sends the message to the body that it is going to be stretching, great!
Remember you need to warm your muscles before you stretch them. That's why Salutation to the Sun is a fantastic way to start a Hatha Yoga class.
But if that's too ambitious; be content with walking on the spot and then doing a series of mini-mobility exercises before stretching.