Williams' system incorporates an electronic device, which houses a compass, at the back of the headstrap on the goggles.

When the swimmer turns, the compass detects that the co-ordinates have changed and registers the time.

The information, including the total time spent swimming, the number of laps completed and the speed traveled are shown in a tiny heads-up display inside the goggle lens, using a system that reflects information off tiny mirrors. [CNN]