As long as you place the force on the sides of the penis, it’s fine. Think of the overhand grip, put the soft bit between thumb and forefinger over the top of the shaft, make the bottom of the grip very loose and put the pressure on from the sides. With a standard grip, think of the same issues, try to not put too much pressure on the top or the bottom of the shaft.
It helps to understand the anatomy. You’re trying to effect the CCs, which are two tubes side by side in a horizontal alignment running through the outer skin. At the bottom the CS feels spongy when you are erect and contains the urethra, on the top you have some major nerves and vessels, so you don’t really want to be putting pressure on these.
The C-grip grips the top and bottom of the CCs, avoiding the centre top and the bottom of the shaft and so avoiding what needs to be avoided.
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