Guys, I need a small explanation on what is the purpose of the -k parameter

Using -k zec zec on my rigs is getting me less speed than with the default -k zec
GPU - Sapphire 480 Nitro OC 4Gb
Anyone cares to explain this?
I tried to use the -k zec zec on my R9 nano, the speed is actually lower. How do you get 40S/s on this card?