Understanding why we feel tension in other parts of our bodies when we manipulate another requires a deeper look into the male-anatomy.
Starting below the rib cage is a group of muscles, commonly called the core of the body. This muscle family is made up of the abdominal structure (rectus adominis, External abdominal oblique, lliopsoas, pyramidalis, inguinial ligament, gludius medeius, thorocolumbar facia, external/internall abdominal oblique, (damn this latin is getting tough, thank GOD I studied it so hard in school

), the penile structure which mainly consists of the lower pelvic floor muscles and some smoothe muscle tissue along the tunica side of the shaft, which embodies the lower back, the pelvic muscles, the penis, the upper gluteus and the extreme upper leg muscles.
This entire protoplasm structure not forgetting the skeletal, ligament and various tissue that shares this space with these muscle groups, are inter-connected via the lymphatic, nervous, cardiovascular systems.
This entanglement, being an incredible wonder of GOD, acts and reacts to every movement we make. when we pull back our penis, as in a
BTC stretch, other muscles are manipulated subconsciously, causing soreness, reaction or even shut down and start up of other portions of the body.
I hope this helps explain things.
Mike (DLD)