A hollow strap-on is a specific type of strap-on that’s created to fit over a penis. Different strap-ons have different styles, extension amounts, and sizes: some are specifically meant to elongate or thicken the penis, while some are just intended to help you maintain an erection. The flexibility of a hollow strap-on helps to alleviate erectile dysfunction, provide an easy way to avoid premature ejaculation, or just make sex a little more fun.