Ambient shakes
A looping shake can belong to a shot. Put one on a Rig and it arrives and leaves with the camera move, as one motion.
A one-shot is an event: two grenades are two grenades, they add together, and changing Rig halfway through should never silence one.
A looping shake can be part of the shot instead. "This is a rumbling shot" is as much a property of a shot as its distance, so it should arrive and leave with it.
Putting one on a Rig#
Give a Rig an Ambient Shake and a weight. When the camera blends to that Rig, the rumble fades in with the same timing and easing as everything else. Walk into a survival-horror Rig and the camera pulls in and starts breathing as one move, rather than the breathing turning up a moment later. Walk out and it settles as the camera pulls back.
Only looping shakes work here. Point it at a one-shot and nothing happens.
Go deeperWhy one rumble morphs rather than two crossfading
Crossfade two looping shakes and you'd run two different rumbles at once for the length of the overlap, beating against each other. Instead the ambient channel runs a single rumble and blends its settings, so its character changes smoothly with no beating.
It also avoids a subtler problem. A rumble normally works out where it is from time multiplied by its speed, which only holds while the speed never changes. Blending changes the speed, so that product jumps and the shake ticks. The ambient channel keeps track of its own progress instead, so it stays smooth through any change.
One deliberate exception: the rumble's speed moves towards whichever side you can actually feel, not to the halfway point. A silent side still has a speed setting, and blending towards it would make an arriving rumble slide up to its real pitch as it faded in.
Go deeperDirector shakes, and trying the blended version
Directors can play an ambient shake too. By default a Director's shake stops and restarts when the next one takes over. Turn on Director Ambient Shake Blends in project settings to have it morph through the same channel instead. It's off by default because it changes the feel rather than fixing anything, so try both on your own content.
One caution in 1.1: with that setting on, a Director's shake and a Rig's ambient shake share the one channel, and the Director's can't take over while a Rig holds it. 1.3 gives every source its own place in the mix.