The balls are a light metal and clang and vibrate non-electronically much like Chinese Medicine Balls. It seems that there is another sphere inside the ball that strikes the inner sides of the ball as it moves.
Both balls are covered in a food-grade silicone that is smooth and odorless. Lint does stick to them.
The silicone band linking the balls stretches easily, and while it was fine in one play session, it seems flimsy enough to break after lots of use. Use extra caution when using anally (although the body will pretty easily pop something like this back out if it's not already inserted past the string).
The two balls are rather large at one and a half inches; for that reason, I did not use them anally. Even vaginally, they are very large. One end of the thin silicone band that connects the balls has a loop for your finger, for easy retrieval. Although a significant amount of pressure was exerted, and the band stretched, it did not break.
Vaginally, the sensation of these balls is interesting. You feel each one as you insert it, and as you withdraw. Thrusting is of course not possible. I found that the most interesting and effective way to use them was to tug rapidly at the handle while the balls were inserted, forcing them to vibrate inside and even clang against each other and putting pressure on the walls and opening of the vagina as they were pulled and then released. It came close to reproducing the sensation of penetration, with pleasant results.
It is very easy to clean and sterilize this product, which you must do not only before sharing with another but also if moving from anal to vaginal play. Spritz with a 10% bleach solution then wash and rinse with soap and water or boil or run in the top rack of the dishwasher with high temperature water.