These are twisting motions you make with your feet, legs and hips to guide your board through turns. When you steer, you pivot the board around a point midway between the bindings. These movements work in concert with edging movements to help you control speed and direction.
As you steer, move your feet interdependently. Twist your front foot in the direction of the turn while pushing the rear foot away from the turn. Try not to push the tail out abruptly. The movement of your front foot should be complemented by the movement of your rear foot to guide the board through a round arc where the tail follows the tip.
English is not my first language and the twist or torsional steering baffled me til coming across a description using body parts. This stuff is also referred to as the "gas pedal", "big toe/little toe", "steering with your leading knee" and "hump and dump". Do this at home on the carpet and you will feel what is described and look closely at the board and you will see the board twisting.
The main thing to focus on imho; is to think of steering the board with your leading knee...its more efficient because it is the largest joint and muscles nearest the board. To steer toeside, just bend your knee and point or drive it toward the center of the turn. You will also notice more pressure on your ball and/or big toe of the leading foot.
To steer heelside, swing your leading knee forward toward the nose. You will notice more pressure on the outside or little toe side of the leading foot and your leading knee will have some slight twisting pressure.
As for other body parts during a turn...you want your body stacked, that is your leading ankle, hip and shoulder to be aligned or staked over each other. So standing on your board in a relaxed athletic posture; your knees are slightly bent, back straight and hips/pelvis sunk into your center of gravity...no stinky butt or bending over at the waist...think of your board as the bottom of a cereal box and generally you keep all your body parts inside the cereal box. To turn you want to slightly weight the nose of the board...do this by shifting your hips sideways toward the nose of the board. So...besides bending or swinging your leading knee, your hips/pelvis are also humping...thrusting when turning toeside.......or dumping...shitting/sitting when going heelside. And for your shoulders need to be parallel with the board and your leading shoulder will dip or lower when going toeside and rise when going heelside.
So toeside...pressure on the big toe, knee driving toward the center, hips shifted toward the nose and pelvis humping with leading shoulder dipping. And heelside...pressure on the outside of the foot, knee swinging or pushed toward the nose, hips shifted toward the nose and pelvis dumping or squatting with shoulder rising.
As for your trailing foot, knee and shoulder, just let follow the leading side.
Frist try to isolate each of these movements, then once you get the feel and how your board responds...start to coordinate and integrate the movements together.