Shell, four-on-four: 1 at the top, 2 and 3 on the wings, 4 in the right corner. X1 on the ball, X2 and X3 in the gap one pass away, X4 two passes away on the help line with one foot in the paint. Why the coach freezes it here: every position in the series is on the floor at once, and all four should be correct before the ball moves.

1 passes to 2. On the flight X2 closes out, X1 drops into the gap, X4 steps up to the gap on the corner, X3 drops to the help line. Why the whole shell moves on one pass: the three positions are defined by the ball, so every pass redefines all of them.

2 passes to 4 in the corner. X4 closes out with no baseline, X2 drops to the gap, X1 and X3 are two passes away on the help line. Why two help defenders now: the corner is the farthest spot from the left side, so both left-side defenders are in the lane; the defense concedes the skip and keeps the lane.

4 throws it back to 2. X2 closes out, X4 drops to the gap, X1 to the gap. Why the pass back is a short slide: the defense was a step toward the ball already, which is what the gap is for.

2 swings it to 1 at the top. X1 closes out, X2 and X3 drop into the gaps, X4 to the help line. Why this is the shell's starting picture again: the rules bring the defense back to the same spots no matter how the ball got there.

1 reverses to 3 on the left wing. X3 closes out, X1 drops to the gap, X2 and X4 to the help line. Why the left side has to be as fast as the right: the shell is run until the slide on both sides is automatic, because the offense will find the side that is not.

Live. 3 drives the middle. X3 slides with him, and X1 stunts from the gap with his hands up, then holds. Why the stunt: a step at the ball makes 3 pick it up without conceding 1 the open catch.

3 picks up and kicks it to 1 at the top. X1 recovers and closes out with a hand high, X3 drops into the gap. Why the recovery is the half of help that matters: a stunt that does not recover has conceded the shot it was meant to prevent.

1 passes to 2 on the right wing while 3 relocates to the wing. X2 closes out, X1 drops to the gap, X4 to the gap on the corner, X3 to the help line. Why the shell never stops adjusting: the ball never stops moving.

2 drives the baseline. X2 rides him with no middle, X4 stunts from the corner, and X3 rotates from the help line to the front of the rim. Why the help comes from the far side: X3 is two passes away and already in the lane, so his step to the rim is the shortest on the floor; the defense concedes the skip to 3 to keep the layup off the board.

2 picks up his dribble in front of the wall with the clock gone and nothing open. Contained. Why the shell is the drill a youth coach runs every practice: this possession was seven passes and two drives, and every one of them met a defender who was already where the rules said to be.