The early days of electronic bike computers/speedometers were pretty interesting, and it amazes me that some ever got to production! The Avocet 20, their first model, was pretty sleek and sophisticated compared to their competitors. The little magnet ring that attached at the hub was easily installed, and the sensor just attached at the dropout. Together, they were unobtrusive and fairly rugged. I'm not sure that magnet ring, with multiple poles per revolution, provided any better accuracy or response than the reed switch that provided one pulse per revolution. That would depend on whether the computer counted the time from pulse to pulse or counted the number of pulses per unit of time. There are advantages and disadvantages to each method.
As Tom notes, the magnet induced a sine wave into the sensor, which is just a wire coil. Not a bad design, since there are no moving parts. The downside was that the circuitry always used power, causing the battery life to be about one year. The competing sensor design was the magnet and reed switch, which seems to be a major factor in the long battery life of computers using them.
I have three Avocets on vintage bikes, and they pretty much do what I want. The main problems I've noticed, other than short battery life, the connectors can be an issue. I've had good results by cutting off the connector sockets at the sensor, and just soldering the wires to the sensor pins. The plating on the connectors at the computer mount is just a tin plating, which wears out quickly. It's best to not put the computer in and out of the mount unless necessary. Fortunately, the connectors appear to be standard Molex contacts, so replacements should be available.
a quick look at the Avocet 20 on my International....
and a look at the sensor and magnet ring at the hub....
Steve in Peoria