I'm a stickler for clean technique and a clean sound. I like to hear every stroke, every tap, every nuance, so I spend a lot of time on <articulation (See March 2 2016) to enhance my control and to refine my sound. And then someone like Stanton Moore comes along and raves about something he calls “the slop”. You can hear it in his playing, especially in his second line shuffle, where the strokes seem to just slosh around joyously. And boy does it cook!
Control is Out
Steve Hackett says that a good guitar solo should sound like it's on the verge of flying out of control, and it's the introduction of 'slop' that makes the thing seem unstable and about to implode, and that makes it exciting. Instability also creates the recurring cycle of tension and release that keeps music moving forward and keeps it interesting.
Using Your Voice
One way to introduce slop is with tuning and voicing. Tuning fairly high with large intervals between drums will improve definition and articulation, whereas tuning low and having your drums closer in pitch will add lots of vagueness, i.e. slop. Lower tuning also gives you a more gutsy, more primal sound. Compare Simon Phillips' clean sound to Steve Gadd's more gutsy tone. Dark and trashy cymbals can have the same effect.
Stick Tricks
You can introduce slop through sticking. Ghost notes, buzzes and chatter strokes can be nice and vague. Any sticking can be relaxed to make it sound freer and looser. You can introduce slop with your execution. Flams and gock shots sound a bit 'dirty' compared to a regular stroke.
Lazy Days
Manipulating the space between subdivisions can be especially effective. When playing a shuffle or swing ride, relaxing the ‘skip beat’ can add a bit of slop, as per Elvin Jones. In Latin rhythms, I sometimes play a pattern that’s roughly half way between 8th notes and 1/4-note triplets that sounds somewhat out of control, and yet is quite funky .
The Bottom Line
Working on articulation will enhance your ability to execute stickings and your ability to get a good sound from your instrument. From there you can easily add slop to your playing. However, if you start with sloppy playing, you'll be hard pressed to call up articulation when you need it.