What you feel in the perineum, between the scrotum and the anus, is the bulb of the penis covered by the bulbocavernosus (also called bulbospongiosus) muscle. This is the muscle that is exercised by doing Kegels. The bulb of the penis is the end of the corpus spongiosum, which is the tube of erectile tissue that surrounds the urethra and ends in the glans (head). It is not connected to the corpora cavernosa, the twin erectile structures in the shaft of the penis. Stroking the BC muscle/penile bulb is like squeezing one of those long thin balloons used to make balloon animals. With a bulge at each end and a thin tube between them, squeezing or stroking one bulb only makes the other inflate temporarily. Personally, I can’t see any benefit to a “between the balls” (BTB) jelq.