Outer Limits 4x4 Board

my parents have a 98 3L diesel jackaroo and on the dash there is a switch that has a picture of an engine with a coil inside looks like a glow plug symbol
when you turn it on aparently it will only work with engine running but when you switch iton it sounds like air escaping but makes the engine run smooth
question is what the hell is it i have not heard the noise it makes but they keep asking me what its for

Sounds like it could be the cold weather idle up (not the correct term)

Yeah, it is a engine heater. It also closes a flap in the exhaust that directs air thru the EGR for quicker warm up. Also ups the idle speed slightly. Once the engine is at normal temp the system shuts down.