its normaly when the percussion pauses for a second and then comes back with a heavier bassline.
it is always very pleasurable to hear one but you really need to hear it on a big sound system to know how good it is.
"yeah...and when the bass dropped"
"yeah it was so heavy"
Scott: "Yeah man, sorry. It's that damn soy milk."
Brennan: "Today at work, I was bass droppin' all over the place."