Slave Cylinder Problem

[Palmstiernas]

Anyone have a 5 speed and have any problems with their slave cylinder? I had a small squeak coming from underneath my IS at idle, in neutral, with the clutch disengaged. Took it to the dealer yesterday and they said it was my slave cylinder. It was mechanically sound but had a squeak. Said they would replace under warranty. I'm just wondering what a slave cylinder is what it does and what could cause (driving wise) it to go bad?