Both the low back and the upper glutes are very prone to compression, congestion, and pain. This move helps roll out a lot of that tension, inviting freshly oxygenated blood to the area. And because you are slightly elevated on the roller, this counts as an inversion (your hips are higher than your heart, and your heart is higher than your head), which decompresses the spine, helps you stand taller, and eases pain.
