Hi there Zhanveeyev darling! Sorry I couldn't respond sooner, but as they say, 'let me talk to my guru first and then I can teach you properly...' I wanted to speak to some of the chiropractors and trainers I work with for some ideas about the issue. It seems like there are two main things that can make that pop: if there is hypermobility at the joint then it can be an issue of bone interaction; but hypermobility is unlikely in your sacral joint where the pelvis meets the spine--it doesn't move much. The other possibility we came up with is just the anatomical positioning of your tissues over the bone. Perhaps a tendon must pass over a bone or joint section in the course of these movements. I'm sure you have tried the spine twist stretch where you lie on your back and bring one knee across your body to the floor on the other side, shoulders on the ground, looking away from the knee... settling into this stretch can facilitate a nice adjustment in those joints. Perhaps when you feel the sensation again you might go back over the range of motion that created it, and if you find the pop-spot, try to gently lengthen and stretch the tendon there, staying in pretty much the same position but lengthening... Sometimes when a muscle or tendon gets tight or shortened it may lay differently across the bones. Anyway, I'm glad it doesn't hurt you! Let us know if you have found out anything more about it!