Actually if you put the magnet so close to the hub that the sensor picks it up twice a revolution it will make you really fast.
This can be a problem when using a Powertap with a magnet, as just a cyclecomputer. ( It has to be close to the sharktooth to pickup, but too close and it picks up twice.)
Makes for an impressive file however.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.