The single mental model that makes doubles click. If you only learn one thing, learn this.
Picture an invisible rope, about 8–10 feet long, tying you to your partner at the waist. Wherever the ball goes, the rope pulls you both. Ball drifts to the left sideline — both of you slide left. Ball comes back to the right — both slide right. You never move alone, and you never stand still while your partner moves.
Two players covering a court independently leave a seam — usually right up the middle — that good opponents will find every time. When you move as a connected unit, you shift that seam with the ball instead of leaving it parked in the center. The player nearer the ball covers the line; the far player slides in to guard the middle. Same shape, every rally.
The rope "snaps" when one of you chases the ball and the other stays put. Now there's a car-sized gap between you and the next shot goes right through it. If you get pulled wide, your partner's job is to slide toward the middle to keep the rope taut — not to hold their spot. Move together or get split apart.
Shadow-drill with your partner, no ball: one of you points to a court position, both shuffle to cover it together, keeping even spacing. Feel the rope. Then add a ball and have a third person feed wide-to-wide so you practice sliding as a pair under a little pressure.