Hoisting sacks sounds like something farmers do, but I'm not quite sure this is it. It's more like scratch the back. Though that doesn't sound quite as inspirational, it gives you a pretty clear idea what to do.
With one bent arm, hold the shovelglove over you shoulder and bend your wrist so that the head is hanging straight down, and the shaft is parallel to your back. Your elbow should be pointing forward. Straighten your arm. Bend it again. Repeat. It's ok if it rubs a bit.
I don't think it matters much if you have the face of the head or the side of the head towards your back, but I prefer face. It's scratchier.
I thought this one up because a musclehead friend of mine pointed out that my routine seemed to neglect triceps. Hoist the sack gets them good.
It's come to my attention that some people do this movement with two hands. Fine by me. It's a little arbitrary that I use two hands for butter churn and just one for this.