There are a number of ways to imagine this one, but here's what I do: imagine you are in the engine car of and old fashioned steam locomotive, stuffing coals into the furnace. Bend your kees, hold the shovelglove perpendicular to your body, and turn so your face and one foot are towards the imaginary oven, the other 90 degrees. Then lunge towards it. Pull back. Repeat.
Another way to think of this movement is "stuff the cannon." Imagine you've got one of those muzzel loading cannons from napoleonic times into which you're ramming powder and shot.
Yet another, also martial, and perhaps even more satisfying way to think of it is "thrust the spear." Thanks John Enzinas, for suggesting this.
Juergen from Austria takes the locomotive motif further and combines this movement with shoveling.
My unscientific pain-based analysis is that this movement hits the forearms and abdomen most.