The basic cadence sensor uses a magnet on the crank, it turns the motor ON when you start pedaling and turns it OFF when you stop pedaling. It works more or less like a switch.
You have to control the boost level and speed by adjusting the assist mode manually up and down. Most basic e-bikes have this.
The advantage is that it's an inexpensive way to get some sort of pedal assist onto the bike, but the disadvantage is that the pedal assistance can feel jerky, laggy and counter intuitive. Also if you want to pedal faster than the motor is spinning, the motor will actively work against your efforts.