Sauna heaters?

Question

Why is it that when the heater in the saunas shut off, there is a sudden surge of heat generated.

First two answers are good. But, I am referring to DRY SAUNAS, where there is no water involved in process.

Answer 1

Not being a sauna designer, I can only guess that perhaps the heater warms the rocks to a predetermined temperature or for a predetermined time, and the controller that shuts off the heater turns on the water flow for steam production at the same time, the idea being not to have heating and water flow simultaneously for fear of an unhealthy amount of steam.
From the ref. (if one wants "steam production at traditional sauna bathing"): "The electronic control logic’s will not begin the steam production until the selected
bathing temperature is reached."

Answer 2

temperatures approaching and exceeding 100 °C (212 °F) would be completely intolerable. Saunas overcome this problem by controlling the humidity. The hottest Finnish saunas have very low humidity levels, which allows air temperatures that could boil water to be tolerated and even enjoyed for long periods of time. Other types of sauna, such as the hammam, where the humidity approaches 100%, will be set to a much lower temperature of around 40 °C to compensate. The "wet heat" would cause scalding if the temperature were set much higher. Finer control over the temperature experienced can be achieved by choosing a higher level bench for those wishing a hotter experience or a lower level bench for a more moderate temperature. Good manners requires that the door to a sauna not be kept open so long that it cools the sauna for those that are already in it. A draft, even if at 100 °C, may still be unwelcome.

water heater uses nothing more than the "heat rises" principle to separate hot water from cold water in the tank.

As cold water comes in, it remains at the bottom of the tank because it is denser than hot water. If you use the hot water faster than the heating elements can heat the incoming cold water, and if you consume all of the hot water that the tank holds, you run out of hot water in the middle of your shower. If this seems to happen too often, it could mean that the bottom heating element in an electric water heater has burned out or that your water heater is too small for your house. Or it could mean that you are taking really, really long showers.