Timex DataLink. I had the Ironman Triathlon branded model of those, used the hell out of it in high school.

The screen flickering trick also only worked on CRTs under Windows 3.1/9x, so if you had a laptop, any other system with a LCD, or were running NT you had to get a special dongle that hooked up to your serial port and would blink a LED that you had to hold in line with the watch's sensor.