Jeun Ma is the power generated in part by the waist but really by the mechanical power generated by rotating your stance.
This is demonstrated very clearly in the Gan Sao/ Baby Huen Sao section of the Wooden Dummy (the little section that is used as a separator between all the other sections).
Wing Chun usually does important things many times in its forms. For instance, the Tan-Wu-Fook section from the start of the Siu Lum Tao which is not only repeated three times but which we are advised to do slowly to really draw our concentration and understanding to the mechanics and body feelings involved.
In the Wooden Dummy, we are always having to use our imagination (since the dummy doesn’t move). This is one of the many reason we call Wing Chun a thinking style. We are simulating fighting in various ways and we need to often bridge the gap between our training and “real life fighting” using our imaginations.
In the Gan Sao/ Baby Huen Sao section, we are catching a right arm strike (a straight punch which is probably accompanied by a step) with a Gan Sao (a Gan or low deflection with our left arm and a Jaam chasing center with our right). The Jaam changes to a baby Huen Sao and we use our stance and hip power to rotate the opponent’s incoming energy away from our center (to our right and past us) as we change the Gan (the left hand) into a strike to the center of mass (probably the head).
The Juen Ma action is sort of like the action of a subway turnstile. The mechanical rotation of our weight supports the deflection of the incoming energy. This is a very important concept in Wing Chun. The “sticking” in this case is really a transient effect of the opponent’s incoming energy and my deflecting Gan Sao meeting (while chasing center) then dropping (the elbow moving toward my own hip) guiding the opponent’s action to my right.
Greg does many variations of this Jeun Ma action in the demo below. Greg steers Tyler’s attacks by meeting them (chasing center) and then deflecting them.