Stances use either inner, outer tension, or no tension (natural):
- Inner tension occurs when the tension in the legs and feet is directed inward toward the center of balance, as in the cat and sanchin stances.
- Outer tension occurs when the tension in the legs and feet is directed outward away from the center of balance, as in the front and sitting stances. Individual stances may use either inner or outer tension, but not both.
- Some stances have little or no tension, such as in front and back stances. Using unnecessary muscles will hamper speed, power, and balance, and cause unnecessary fatigue and a lack of control.
Some instructors speak of squeezing the feet into the floor while creating leg tension to strengthen the stance. Some claim this acts as a suction cup to make the body more solid on impact so it may resist any attacking force by using friction.
Friction is the interaction between two (or more) surfaces, expressed as a coefficient of friction. It relates only to the type of materials, the smoothness if the surfaces, and their geometry. Since you cannot change the composition of your feet or the floor, or the coefficient of friction, you can only change your geometry by maximizing your foot to floor contact, so do not curl toes, or rise on balls or heels.
How effectively you use that friction is the key. Friction accounts for the feet sticking to the floor, but it does not account for the feet being pushed outward or inward against the friction of the foot-floor contact that creates extra force into the floor. This does not happen simply because of gravity since the force of gravity and your mass are constant. If you exert a downward component of force greater than that used to support your body weight, you will rise. Since you do not do this when squeezing your feet together or pushing apart, there is no vertical component of force. Therefore, any squeezing should be done horizontally, not into the floor
This squeezing stiffens the legs but whether this actually strengthens the structure of the stance or provides stability is debatable. Some would argue that the increased muscle tension creates a more stable structure. Others would argue that the tensions counter act each other in opposite directions and accomplish nothing. Others argue that too stiff of a stance structure may actually weaken the stance, and that pushing against the friction of the floor contact continually is actually reducing the amount of force necessary to break that contact to the floor. Until formal research is performed and replicated, no one will know for certain.