How the Mid-Drive eBike Technology Works:
Figure 1 - The Mid-Drive Technology
The figure above is a representative figure to explain how the geared mid-drive technology works. In the figure above, you can see a drive sprocket. That drive sprocket drives the chain through a motorized mechanism inside the grey box. In essence the chain is pushed by the chain wheel (you) and the motor (Drive Sprocket). In the above arrangement, there can be a number of additional elements to “enhance the riding experience” such as a speed sensor on the wheel, a torque or crank sensor etc. (What is the difference between a torque and a crank sensor? - This is another important thing to learn. What are the different torque sensor technologies and how do they affect your riding experience)
In that grey box in figure 1, you have a small motor which can produce a torque of no more than 2-4Nm spinning at a high speed. And then through multiple gear stages, the speed is reduced so that, finally it matches your crank speed (when done correctly). So for instance, if you have a motor running at 2000 rpm with a torque of 2Nm, we could potentially reduce the speed to 100 rpm through gearing and increase the torque to 40Nm. Nowadays, Mid-drives are boasting of torque in excess of 120Nm in certain cases.
Takeaway: Mid-Drive drives your chain.