Because Biped's keyframe interpolation is based on the mechanics of two-legged characters, it varies according to whether the feet are on or off the ground. When you activate footsteps, the default keyframes for the leg tracks are given states that correspond to the footstep pattern from which they were created.
A leg's motion has four phases, beginning with the foot on the ground. Then the foot lifts, moves through the air, and returns to the ground again. Biped divides this motion into four phases as follows:
Touch occurs at the leg keyframe where the leg's foot first touches the ground and always corresponds with the start frame of a footstep in Track View.
Plant occurs after touching, and before lifting. It is always in between the start and end frames of a footstep in Track View.
Lift occurs at the keyframe where the leg's foot lifts off the ground and always corresponds with the end frame of each footstep in Track View.
Move occurs while the foot is in the air and is always in the intervals in between steps in Track View. In walking, while one foot moves the body is supported by the other leg. In running or jumping, while a foot moves there is a period where the body is not supported, and moves in midair.
Biped displays the state of each foot in the General rollout of the Motion panel.
Regardless of the footstep pattern, in order to properly interpolate you character's motion, Biped will always generate and maintain leg keyframes at each lift and touch transition in the steps.
Footstep states in General rollout
For our purposes, a walking gait is defined to be any footstep pattern in which there is always at least one foot on the ground. Of course, this is a more general use of the term since it includes many irregular and non-periodic footstep patterns, and even standing in place. The main feature of walking gaits is that there is no dynamics of ballistic motion at work.
When Biped first activates footsteps to create default keyframes for walking footstep patterns, it attempts to create as few keys as possible to establish a fundamental framework for you to work from. In general it will create:
Lift and touch leg keys at each support transition, several plant leg keys to both flatten and pivot the foot, and two move leg keys, one in the middle of the swing of the leg, and one to straighten it out just before its heel strikes.
Vertical keys at each support transition and one in the middle of each support period.
Keys for other body parts according to the rhythm of walking.