This has been covered many times in this forum...
Basically u use one sound 2 make another duck!

so for instance u can have a kick drum make a bassline dropback it volume a little everytime u have a kick drum... if donne right u get sort a groove/ pumping feeling in yer Bassline and also u can get a more punchy/ "transparent" kick that wil cut thru the mix like a charm while at the same time having a loud bass without one cuting of fighting with the other for sonic space!

Z