The tunica is a thin, dense sheath that covers the shaft of the penis, just below the dermal level. It is very difficult to stretch due to it's lack of elasticity (only about 5%). It is believed that upward stretching is best to stretch the tunica but I believe it is wiser to stretch internal penis, as seen in E
xpressive Stretching) because it is always easier to pull new penis from the body then stretching the tunica. It can be done with much work but usually the gains are not worth the effort.