Short or small bladed swim fins, such as Zoomers, allow you to maintain a foot speed closer to your regular kicking speed with no fins. They also add just enough extra surface area to give you more power from the kick. Zoomer-type fins come in two colors, blue and red. The blue fins are made of a softer material for folks new to short fin kicking or those that have a less efficient kick. They are less stressful on the ankle due to a more flexible fin blade, much more comfortable when starting out. The red are stiffer fins, resulting in more force per kick, but that also results in more stress on your joints and muscles.
You can make your own short blade fins by cutting off all but two or three inches of the fin's blade. The home-made variety are usually soft, somewhat comparable to the blue Zoomers-type fins. The advantage of Zoomers is their finished edges and consistent quality. Other short blade fin designs comes from a variety of sources.
Medium to long bladed fins offer more power form each kick, but potentially at the expense of foot speed. They are great for working on dolphin kick and butterfly. You can feel your body and leg movements as you swim - bigger fins add emphasis, amplifying each kick. One good brand of medium bladed fins is the Churchill, featuring a blade that is not too long for competitive swimming.
Avoid "very long" or vented, scuba style fins. While excellent for scuba uses, they are not the best choice for a swimming workout with fins. The fin's length and design result in very slow movements, too slow to give you as much specific benefit as short fins. The long fins still offers some benefits, for flexibility, increased workout load, and speed, but not as many as the shorter or medium bladed fins.
And then there is the monofin, single-bladed swim fins. These fins are also a great tool for workouts, particularly for developing strong legs, abdominal and back muscles, and working on butterfly technique.
There is an official sport called Fin swimming. Fin swimming has competitive events where athletes wear single bladed fins (called a monofin) and race either under or at the surface for various distances. These races are fast! The records for 100 meters with a monofin, as of 2003, are :40.74 (surface) and :36.26 (underwater or apnea). Compare that to the World record for 100 meters swimming of about 47-seconds.
Among the things that you can gain by using fins is improved ankle flexibility from the extra force the fins place on your ankle as you kick. Increased ankle flexibility will result in a more efficient flutter kick through better angles of attack on the water.
One of the greatest benefits of using fins is the ease of holding a better body position. This allows you to focus on other parts of your technique, such as body roll or timing. You should add fins to your workout kit, along with the rest of your swimming toys and tools. They have a lot to offer to make you a faster swimmer! Let me know if you give them a try.