The breakdowner is a thing of beauty working not only bones but object(s) too – it automatically calculates the values of the prev and next keys from the playhead, without the need to select anything. It works in in the Graph Editor, Dope Sheet and Timeline too.

Pressing the Prev, 1/2 Mid and  Next keys give a preview of those values, here the Prev button and Key L+R+S buttons have been pressed.

Pulling the slider into negative here has produced an overshoot from the prev key –

See it in action here