In The Book of Wing Chun, Vol. 1, pp. 52-53, 7 powers are listed as a sequence serving as the Order of Using Different Powers. They are: 1) Idea Power; 2) Moving-body-weight Power; 3) Rotational Power; 4) Shoulder Power; 5) Shoulder Joint Power; 6) Forearm Power; 7) Wrist Power.
This would be best understood against the backdrop of the skeletal system, with Power (1), initiating every single movement of the body (refer to the post [The Skeletal System Takes Over the Master Role] dated 2014.01.20).
Powers (2) and (3) are the working of the spine initiating the moving of the body – away from the body’s original position for (2) and setting the body to pivot for (3). Power (4) results from the working of the rib cage which then moves the entire shoulder joint. Power (5) refers to the working within the shoulder joint which rotates the upper arm to give out the commonly known Elbow Power. Power (6) comes from the working within the elbow joint that navigates the forearm to achieve focusing (aiming at) rather than pushing or pulling. Power (7) refers to the working within the wrist joint (the smallest among the above) that activates the palm and fingers, and is instrumental in manifesting the power as Sending-power-to-fingertips in Darting Fingers (Biu Jee).
“[T]his sequence also represents power generation from low to high, analogous to car driving: to attain the speed at the fifth gear, one should always start from the first gear and change gear levels upwards one by one.” (p. 53)
The 5-gear analogy is perhaps more matching when discussing the Mechanics of Darting Fingers (pp. 225-229), in which exactly 5 gears (powers) are listed: (1) Moving-body-weight Power, which is the strongest; (2) Rotational Power from rotations around the centre of the body as the axis; (3) Propelling and Drawing Powers of the shoulder; (4) Rotational Power from rotations around the shoulder joint as the axis; (5) Power from the movement of the forearm.
Following the line of thought for understanding the 7 powers above, it is not difficult to apply the same interpretation to these 5 which are highly similar. (The omission of Wrist Power here, I think, is due to the analogy being applied to the typical elbowing movement, in which the activity of the wrist is not obviously seen. However, as far as the gear analogy is accepted as appropriate, you can simply increase the number of gear levels whenever necessary. That is, add Wrist Power as the 6th gear in a 6-gear analogy.)
There is one major difference deviating from the analogy, though: “Nevertheless, in vehicle gear operation, when changing from the first gear to the second to the third and so on, the solid power of the first gear will disappear accordingly; whereas in Darting Fingers, all of the five gears coexist and can operate simultaneously.” (p. 228)
In vehicle gear operation, at a time only one gear exists and can only give out its associated speed/torque trade-off combination: lower gears output higher torques but lower speeds; higher gears output increasing speeds but with diminishing torques. In Darting Fingers, “the five gears coexist”. Both power from torque (at lower gears) and power from speed (at higher gears) coexist without diminishing and together take effect at the same time on every movement.
No wonder Wing Chun movements are both extremely fast and rich in power!