Variation/Progression: Progression is usually by increasing number of repetitions. But it is also possible to use dumbbells, around 1-3kg, handheld either side of the neck.
Another very useful increase in intensity is to lean, and hold the pose for a count of ten before returning to the vertical (increasing as you get better at it). This is close to the timing and muscle use needed for field archery on medium steep slopes, so it trains for uphill and downhill shooting.

Primarily abdominal obliques and spinal muscles.