A hi-beat (or high-beat) watch is one that beats at 36,000 vph, or 10Hz. First introduced in the waning days of the sixties, they are harder to make but can be more accurate. The higher rate causes all sorts of problems, though, so they're quite rare.
According to the 36000bph site, there were nine efforts to make a hi-beat movement. Good read.
- Grand Seiko has reintroduced them
- The Zenith El Primero (chronograph) is hi-beat. (This was the movement in the Rolex Daytona until 2000, when they introduced the Rolex 4130.)