The opening is so narrow that it requires a really hard erection to get the penis in. For us older guys (pushing 70), a really hard erection is sometimes a problem.
Once inside the very tight hole, it grasps the penis incredibly firmly. I was only about 75% "rigid" and because of the 'thrusting' force necessary, I was only able to get about 1/2-way in before I had my orgasm.