When I was 14 or so, we did our first studio recordings. We went to Memphis (and got to use the same board that Zep III was recorded on!) and after a couple of takes the producer took outside to the sidewalk and asked the drummer and I, "How long you been playin?" I said "About a year and a half, two years," and the drummer said about the same. He nodded and asked, "Do you know what it means to 'lock in' together, as bass and drums?" We both shrugged and he explained to us, "When the drummer hits the kick, you should hit a note. When the drummer hits the snare, you may want to hit a note. And if the drummer is doing neither, find out what the hell the drummer is doing and do that, too."
We took it to heart, and every single beat from the kick or snare was duplicated verbatim on bass. Of course I had my own things, and he had his own fills, etc., but as time went on and we got better those fills began to lock in beat for beat, note for note, tom for tom, through the entire roll. It didn't help that we were huge Rush fans, having discovered them when "Farewell to Kings" came out - it was Geddy and Neil, more than anyone, who lock in phenomenally in the same way. "Cities full of hatred, fear and lies..." That's it.
Fast forward to now. I'm privileged to play with an incredible drummer who is a Portnoy devotee, playing all of Transatlantic's catalog, most of Dream Theater's, all of Neal Morse and a good bit of Rush. His masters is in music and he teaches music full time, playing several instruments. We've been playing together for 7 years now, and are as tight as we can possibly be, meaning that when I pop a fourth lower of a root for emphasis and do it very quickly it's almost intuitive that he does it too with his kick (as found in Cinderella Man, for cryin out loud!) Note for note.
A cymbal may get picked up as a harmonic pop that hangs over a riff, the closed high hat might get duplicated as a high fast tapping thing or whatever. But if his kick is pulling triplets, the bass will be right on top, playing triplets, not straight time, even though that might line up (eventually). Now of course there are places without drums but have bass, and vice versa, and places where we pull back and funk it up a bit with some improv, but we've played together so long it's almost presciently doubled anyways). We go to great pains to insure that every fill on the drums is matched note on the bass (where and in what technique is up to me, but if he doesn't like what I'm doing I'll usually toss it), so I have to double very fast fills, and some with crazy splash cymbals or a timbale thrown in here or there. Keeps me on my toes.
Just last night at rehearsal, we were playing one of our new originals and he wanted to change something so that instead of JUST having the drums hit a whole note in the intro, the bass would (ta-da!) double it by hitting E on the 12th and dragging down to G on the 3rd in time and then fast muting. So we're still doing stuff to double one another.
There are places where the bass doesn't follow the drums but follows the guitar (Dream Theater's "Erotomania" for instance) and then locks back in with the drums in an unexpected way, and of course, lots of places where the band all lock in together for crazy stuff (again, Dream Theater comes to mind, and some of our originals as well).
But generally, the way it was explained to me all those years ago still fundamentally holds true: "When the drummer hits the kick, you should hit a note. When the drummer hits the snare, you may want to hit a note. And if the drummer is doing neither, find out what the hell the drummer is doing and do that, too."
That's lockin in to me. YMMV.