Maintaining the half sit-up position while performing any movement with the arms will challenge the abs, hip flexors, and even the neck musculature.
Anchor your feet under something stable and lean back until you feel your abs turn on. Try to keep your spine and neck as neutral as possible. You can go light and do the exercise for 60 seconds, or use heavier weights and shoot for 10-15 reps.
The "lumberjack squat" isn't what people in the logging industry head off to do behind a large poplar tree after having a hearty breakfast of bran flakes. Instead, it's a helluva' cool squatting movement that doesn't compress the spine.