Ralf,
Heaving to- meaning to slow the boat down can be used for a number of reasons. It is usually done by backing a headsail, with or without any kind of mainsail up and lashing the tiller to leeward. This slows and steadies the boat. As she tries to respond to the rudder by heading up, the headsail pushes her back down. While this is going on she will make 1 to 2 knots to leeward.
Some reasons to do it: STORM CONDITIONS- When it is too rough to carry on and you need a rest. Heaving to under these conditions creates a slick to windward helping to settle the waves to windward. Once the seas become too large and dangerous it is time to run downwind.
ARRIVAL AT DESTINATION- Arriving at an unfamiliar destination when dark, heaving to is used to wait for daylight.
ARRIVAL AT ANCHORAGE- Approaching an anchorage heaving to gives you time to prepare your ground tackle.
MAKING REPAIRS AT SEA- Heaving to steadies the boat while you fix what needs fixing.
WHEN COOKING A MEAL- Heaving to keeps you from wearing your scrambled eggs on your feet.
As with normal sailing the wind conditions determine what sails to use. It is truly amazing how exhausting it can get out there. Heave to, get some rest then carry on. Of course if you are racing you just have to carry on.
Lee
I should add for those that do not know under storm conditions in the northern hemisphere heave to on the starboard tack. In the southern hemisphere heave to on the port tack.