Not all cadence sensor software is created equally, so there can be quite a difference from a practical stand point from one bike to the next. This is one of the reasons you'll hear that riding a bike before buying it is a good plan.
P-38 has done a nice job describing "speed" based cadence software. You can see that the different PAS (pedal assist) levels must be changed to change your speed.
The "other" major type of cadence sensed PAS is WAY different, and offers a much more normal experience to my way of thinking. It does this by allowing a certain amount of power to the motor, depending on PAS level chosen. This amount of power doesn't change at ANY speed. If you want more "assist" you dial up the PAS level and it supplies more power to make it easier to pedal. You want to "ghost" pedal with no effort? Set the PAS as required.
For instance - in PAS 1 my bike will have less than 100 watts available for the motor to assist you. It actually varies if you look at the watt meter, bouncing around from about 65 watts up to 100 (keep in mind the controller uses about 50 watts for internal housekeeping), but you feel none of this. It's super smooth, with no jerky on/off as your speeds change. Lets say you make a corner, and the wind direction changes to something hitting you in the face, making it more difficult to pedal. Bumping up to PAS 2 doubles the available power (to 180-200 watts or so), cutting the effort to maintain the SAME speed you were going by 1/2. Hills the same way, or maybe the pavement turned to grass. Set the PAS level for the effort you want to put into pedaling - up to level 5, and it just adds the power you want - no more - and speed has nothing to do with it. Going faster takes more effort, so you dial up the PAS level to reduce the effort to what you are looking for.
I'm going to get into this conversation because I tested this myself on my Espin Sport and a cadence sensor.
At any PAS level, there is a maximum amount of power available. While in each PAS level, how much of this power goes to the motor is inversely controlled by the cadence of your pedaling. Pedal slow, more power, pedal fast, less power.
Additionally, there is a speed limit at PAS. What this does is if you are going faster than the limit at that level, the motor only gets nominal power because there is no need to give you 100% of the available power.
To illustrate this, let's say PAS 1 gives you 100 watts (on a 500watt motor) and has a limit of 10mph.
You start pedaling, 100 watts goes to your motor because you are pedaling slow (this is why on many cadence sensor bikes there is that "jump" when you first start). Then as you pedal faster, that 100watts become 50 watts or 25 watts because you don't need that much assist. Now, say you go down a hill and exceed 10mph. The motor will only get 10watts because you are over the speed limit. This makes sense because why supply power to the motor if you are going faster than that PAS level. Even if you slow down your cadence, the motor will not receive more power until you are below the 10mph limit.
Does this sound right?