Look up Sweetspot in the Glossary of the Golfing Machine. Essentially the lengthwise center of mass of the golf club. If you let a golf club hang from the grip end and also have a weighted string hanging from the grip end - the string would show you the longitudinal center of gravity of the golf club and the point in which it passes through the clubface is the sweetspot. If you tap on the face to the left or right of the sweetspot - everything will rotate around that longitudinal center of gravity.
As Mike correctly pointed out, hang your Plum Bob (line) from the point where your #3 Pressure Point would be and the weight will pass through the Sweetspot which is a pin point (contrary to what club manufacturers tell you). The Sweetspot LCOG is On Plane. The clubshaft, however, moves from its own Plane to the Sweetspot Plane and back to its own Plane again. So the Clubshaft Plane is always shifting between its own Plane and the Sweetspot Plane. This can be a little confusing.
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