Mingmen, the Gate of Life (AKA Du-4) is located in the center of the spine, just below the 2nd lumbar vertebrae. It’s used for building Yang, treating infertility or sexual issues such as erectile dysfunction, and for lower back pain or weakness.
Be careful stimulating this point if you are using acupressure, since it’s often tender. Applying heat can be useful, especially If you tend to be cold. In my clinic, of course, I use acupuncture and an infrared therapy lamp. At home, you can direct a warm shower stream onto it or use a microwavable rice bag (I have them at the office if you need one).