Hamax child seats are attached to the bike via a two-pronged metal frame that slots into a seat-tube-mounted anchor point. This set-up doesn't require a rack, so it's a good choice where fitting one isn't an option.
It also offers a smoother ride for your little one, because the seat is suspended on its sprung frame rather than directly attached to the bike. The one harness is adjustable for height and length, with a secure (if slightly fiddly) closure, and there are footrest straps too.
Side protection, though not as extensive as other seats such as the Co-Pilot range, is fairly good. The Sleepy also reclines via a twist knob, so it's a good choice if your child has a tendency to nod off on the bike - hence the name. In use, the sprung system is a little disconcerting at first but you soon get used to it, and on balance it puts less of a strain on bike (and rider) than a rack-mounted seat. Removing the seat is easy, and the anchor point is unobtrusive when the seat isn't attached. Bikes with unusual seat-tube angles may require a different seat frame, available from the manufacturer.