Actually, sheeting and/or traveling in increases the angle of attack. The goal being to increase the angle of attack to the point that the flow separates from the lee side of the sail and causing it to stall- resulting in a large decrease in power. The problem with this technique is that if the wind direction moves forward or the boat heads up to the point that the flow "re-connects" you will experience a large, rapid power increase.

If you bear off too far, the airflow will switch to clew first- i.e. sailing by the lee. It's doable, but not stable, so the potential for a flying gybe is pretty high.

After all depowering that can be done through tuning is done, reducing sail area is the safest and most effective next step towards depowering.

sm