We've been working on a scarf animation, which is kind of like a series of bones swinging back and forward, but each child bone is delayed from the parent.
I've attached a picture that will hopefully help explain things.
Creating the red key frames for the first bone is easy enough, just a key for the left and right positions and use a curve to interpolate between them nicely.
The problem is with the green child bone, you can easily copy the frames from the parent and offset them, but now at frame 0 (orange line) there is no key frame. Here you'd need to go in and calculate what it should be, which is a bit of a pain and would need to be redone manually if you wanted to adjust things.
I had an idea that if we could set key frames in a negative position on the timeline then that would simplify things.
We eventually went with computing the rotation with a sin wave every 5 frames in an excel file, and linearly interpolated between those. From there it was easy enough to offset the children along those 5 frame multiples.
I suspect there is a better way to do that, but it worked for us 😛