Ganchu is one of the "Vital Points of Chudan (Middle Row) and Spots to be attacked" contained and described in Hohan Sokens notes. It is a Fatal Target - "Causing stoppage of lung mechanism and breath and blood circulation".
For many years it was supposed that the actual target was ST-18 (acupuncture pint at the base of the breast, but below the nipple. This area is to far removed from the heart as well as laying on a structurally stronger area of the ribs. It is difficult to get great affects of incapacitation or dysfunction on the ST-18, however when the correct anatomical target is properly accessed the target becomes a possible fatal attack.
To learn the method, targets and see the affect in real time, the extended films are located at these links: