They are also known as contactors, in the UK at least. It is a device with 2 plugs and an extension socket. One plug goes straight into the mains, and the other plug goes into the timer. Your light plugs into it's extension.
When the timer switches on, it switches the load on the second plug, to power on your light. By using 2 seperate plugs, the surge that is created by the HPS ballast does not have to pass through the timer at all, preventing sticking timers, and fires.
A word of warning to people who are going off the rating on their timer, and thinking that they are safe. The inductive load of a magnetic HPS ballast can be up to 20 times the normal load. This means that, for example, a 400 watt HPS might seem like it would be fine on a timer that is rated at 3000 wats, but the startup surge may be up to 8000 watts, which can lock up or burn cheap timers easily.
People may say "I have used one for years, and never had problems", and whilst that may be true, it is still a very real risk, and easily avoidable for a small cash outlay.