The term 'clipless pedals' is misleading because it refers to pedals that actually clip to a cyclist's shoes. The origin of the term comes from the infancy of clipless systems, which replaced the cagelike baskets called toe clips that allowed the rider to both pull up and push down on the pedals, improving pedaling efficiency. Because the term 'clips' was already in use, clipless pedals got their name because they attached the pedals to the rider's foot without toe clips.
Clipless pedaling systems have three components: the pedals, cleats and shoes. When the rider steps down on the pedal, a spring-loaded mechanism in the pedal similar to ski bindings clamps down on the cleat mounted in the sole of the shoe. Cycling shoes have rigid soles, so when a rider is clipped in, the entire sole of his foot becomes a platform to push the pedal. To disengage the pedals, the rider rotates his heel outward.
Shimano, Look and Speedplay all manufacture clipless pedals. Shimano designed Shimano Pedaling Dynamics, or SPD, cleats for mountain biking. SPDs fit all mountain bike pedals, whose design allows them to shed mud easily. Most SPD-compatible shoes have a recessed cleat and slightly more flexible soles to make walking easier. Shimano also makes road cleats that compete with Look. Road cleats are larger and cover the ball of the rider's foot to give a larger area for power transfer. Road cleats protrude from the sole of the shoe, making them harder to walk on. Speedplay pedals offer more 'float' or side-to-side rotation than other road pedals.
Because a clipless system transforms the entire shoe into part of the pedal, it allows for more efficient power transfer from the rider's muscles into the bike. Also, because the rider's feet are fastened to the pedals, she can pull up on the back portion of the pedal stroke when she climbs a hill or accelerates. Both of these advantages of clipless systems lead to faster, more efficient riding.
Most new clipless riders fear that they will topple over because they won't be able to disengage from the pedals when they stop. Unfortunately, most clipless riders have had to endure this indignity at least once. To avoid this, ask your bike shop to set your cleat tension as loose as possible until unclipping becomes second nature. Before taking your new pedals for a test ride, practice clipping in and out on a soft surface or on your indoor trainer.
When you are buying a clipless system, consider what kind of riding you will be doing, suggest Bicycle World, a Houston-area bike shop. Will you be walking in your bike shoes a lot and thus want a recessed cleat? Will you be riding on the trails or road? Bikers who ride often or competitively may decide to shell out the extra money for lighter, more durable pedals. However, if you are new to cycling, the base models will probably serve your needs. Most studio cycling bikes are compatible with SPD cleats.