A useful exercise to 1) gently mobilise the lower back 2) articulate the lumbar spine 3) allow the smaller muscles around the spine to stretch and gently tone them 4) strengthen your core musculature 5) help local drainage to allow for fresh blood to heal local tissues.
Particularly helpful when your lower back is very sore and acute.
Aim to use a firm surface, for example a thick carpet or yoga matt, as a bed may be too soft. 5 minutes twice daily is a good place to start, unless instructed otherwise. The movement shouldn't be painful. If it is, decrease the range of motion to stay in a pain-free range.
To do at your own risk. I cannot be held responsible for any injuries sustained whilst doing this exercise if I am not in the room with you to supervise.