i went to a few camps a couple of years ago, and the staff repeated over and over "it's not a bicyle step, it's a re-articulation of the foot."
on the "and" count the knee breaks the plane of motion by bending while keeping the toes on the grond and lifting the heel. on the "one" count the heel hits the ground and the toes are as high as possible. on the "one and" count the right foot should be perpendicular to the ground at the ankle of the left foot (with bent knee). and on "two" the right heel hits the ground.
the interesting part was the backwards technique, where the initiation is the same as forward, the left food breaks the place of motion by bending the knee and keeping the toes on the ground while lifting the heel. on "one" the foot is placed behind you. on "one and" the right foot does the same thing as going forward, perpendicular to the ground, at the left ankle, knee bent.
the staff said that if you took a picture on the "and" count, you shouldn't be able to tell if they are moving forward or backwards.
sorry to any cmm, or fmm if this is completly wrong, it's only what i remember from a few years ago, and only going to a few camps (then marching two years in another corps with a different technique).