When performed properly, the float serve is effective at every level. The ideal float serve should not have any spin. It was termed the float serve because the ball should bob and weave as it travels; essentially, "floating" through the air much in the same way a buoy would on the open ocean. A well-executed float serve can be extremely difficult to pass because of its unpredictable nature.
All of the following instructions are for a right-handed server. Simply reverse if you are left-handed.