Old thread, sorry.
As a person with a bad back and then riding b******k & saddled, I can easily pop my sacrum. Most common now after a good horseback riding session.
I pop it a few different ways. Easiest is when laying in bed face down, tighten legs upward from b*m down to heels, while rest is loose.
Same with when standing. Hands on back of hips, back and legs relaxed, push hands down and onto the area. Others are random and I can't reproduce it well.
I can also pop my sternum, anyone else? ?