Swim fins are available in both open heel designs as well as closed heel designs. Open heel fins are generally more popular than closed 'full foot' fins due to the preference of many users to wear booties which are warmer in cold water, and booties are more comfortable on hot dive decks and also safer for shore diving while walking on rough shore terrain prior to entering or exiting the water during snorkel and scuba diving. Also, open foot or open heel fins may be adjusted to exactly fit the user's feet. Closed heel fins are usually less expensive and generally worn barefoot or with socks and have high comfort if properly fitted, as well as having less drag resistance than open heel fins while kicking or moving through water.

Efficiency of swim fins is a function of the average forward velocity in water and the power required to achieve that speed. The diver must overcome drag while swimming, where active drag (drag from a diver swimming through water) is generally greater than passive drag (drag from a diver being towed through water). In order to minimize active drag while flutter kicking for example, short kicks are advised over long kicks in order to minimize the projected area profile of the diver. However, to further confuse the matter, greater energy may be required to perform short kicks at high frequency versus long kicks at a lower frequency because moving a small mass of water rapidly is less efficient than moving a large mass of water slowly; so efficiency comparisons between long stroke kicking versus short stroke kicking are not apparent.

During high frequency flutter kicking Truefin performs exceptionally well and high swim velocities are achieved. Whereas during rapid kicking traditional flexible fin blades collapse (or 'go flat' as it is known in the industry), and for this reason very stiff traditional fins have been chosen in the past in order to minimize the likelihood of the stiff fin blade collapsing during moderate to high kicking frequencies. Furthermore, a disadvantage with a traditional stiff fin is that muscle fatigue and relatively high oxygen consumption for the amount of speed achieved occurs during low kicking frequencies with a stiff fin, and as a result a traditional stiff fin is inefficient and uncomfortable to use when kicking at slow to moderate speeds due to abnormally high strain at the user's ankle for the low amount of thrust generated. Furthermore, at low kicking frequencies, with a traditional stiff fin much energy is lost with water spilling over the sides which increases resistance and produces no useful work. If a traditional fin is considered stiff enough that it can not be 'over kicked', then at low kicking frequencies the fin will perform unsatisfactorily and may cause muscle cramps.

The fin blade flex characteristics of Truefin equipped with Blue spines is twenty degrees (20°) in the 'toe up' direction, and sixty degree (60°) in the 'toe down' static flex direction (45° 'toe down' effective dynamic flex) and is determined by the articulation limit between each of the collision sites between a series of artificial vertebrae. In the Blue spines the articulation limit between successive vertebrae is four degrees (4°) in the toe up direction and twelve degrees (12°) in the toe down direction.