In the study, 30 people used a finger on their right hand to touch a finger on their left hand by tapping a device place directly over the left finger and could instantly relay the tap. The computer-controlled device could introduce delays of varying length before the left finger was tapped. Researchers used another button to introduce externally generated taps.
Based on the test subjects' reports of what they felt, the sensation in the left finger was less during window of time centered on the instant any self-tapping would have occurred naturally.
Bottom line: When their brains expected a tap and the tap came as expected, the brain noticed it less.