From Mechwarrior Living Legends Wiki
Heat Sinks, or HS's, are specialized radiators designed to disperse the excess heat caused by Weaponry, Equipment, and Activity. Each unit typically has several Heat Sinks when neccessary, allowing it to disperse any heat generated more quickly. Note that HS's do not increase the amount of heat your unit can take, they simply increase the speed at which heat is removed.
Heat Sinks are most effective when the unit is Powered Down; also, unit movement speed can affect the rate at which heat is dispersed: the faster its moving, the slower heat dispersion is.
Overheating can cause damage to heat sinks. Damaged heat sinks drastically reduce an asset's ability to shed heat. Continued overheating may destroy a damaged heat sink. While damaged heat sinks can be repaired, destroyed heat sinks cannot.