I believe bike computers use reed sensors.
You can hear the reeds contacting. Try moving the magnet back and forth over the sensor in a quiet room, you'll hear the sensor clicking.

From reed-sensor.com:

FAQ on Reed Switches and Reed Sensors
What is a reed switch ?
The basic reed switch consists of two identical flattened ferromagnetic reeds, sealed in a dry inert-gas atmosphere within a glass capsule, thereby protecting the contact from contamination. The reeds are sealed in the capsule in cantilever form so that their free ends overlap and are separated by a small air gap.



What is a reed sensor ?
A reed sensor is a device built using a reed switch with additional functionality like ability to withstand higher shock, easier mounting, additional intelligent circuitry, etc.

How does a reed switch work ?
When a magnetic force is generated parallel to the reed switch, the reeds become flux carriers in the magnetic circuit. The overlapping ends of the reeds become opposite magnetic poles, which attract each other. If the magnetic force between the poles is strong enough to overcome the restoring force of the reeds, the reeds will be drawn together.

What magnetic force is required to close a reed switch ?
Minimum force - expressed in ampere-turns, will cause the reeds to close, and is called the just-operate force. Since the force between the poles increases as the gap decreases, a force of approximately half the just-operate force will maintain the operated state. Speed of operation of the reed switch is determined by the excess of operating force over the just-operate force.

What are the advantages in using reed switches ?
They are hermetically sealed in glass environment, free from contamination, and are safe to use in harsh industrial and explosive environments. Reed switches are immune to electrostatic discharge (ESD) and do not require any external ESD protection circuits. The isolation resistance between the contacts is as high as 1015 ohms, and contact resistance is as low as 50 milliohms. Reed switches can directly switch loads as low as a few micro-watts without needing external amplification circuits, to as high as 120W. When used in combination with magnets and coils, they can be used to form many different types of relays.

So basically that's the gist of it.
While it's never been confirmed, if the contact is closed when the magnet passes over it and you park the bike with the spoke that has the magnet directly over the sensor, that might drain the batteries a lot faster in the computer or transmitter (if you have wireless).