Opening the book…
People grow at the edge of what they can already do, and they only step onto that edge if failing there isn't fatal. Assign well within range and you get reliable work and no development; assign far beyond it and walk away and you get a preventable disaster plus someone who now doubts themselves. The pairing — stretch plus proximity — is what makes the risk worth taking.
Pick the next rung, not the top of the ladder: the presentation they've only watched, the client call they've only sat in on. Tell them plainly that it's a stretch and that you'll be nearby. Then check in on a schedule you set in advance rather than hovering, and let them make the small mistakes that don't need catching.
Don't stretch someone on work where a failure damages a customer or a colleague who didn't consent to the experiment. And when a person is already at capacity or fresh off a hard failure, consolidation beats stretch for a while.