When may I stand behind the player before they hit?

The restricted area is on or close to the extension of the line of play behind the ball. The limits apply from the moment the player begins taking their stance (at least one foot in position) until the stroke is made:

• To help with aiming: never. You must not stand there to help line them up. This includes moving away silently as a signal that they're aimed correctly.
  • No penalty if the player backs away, and you move out of the restricted area before they start taking their stance again.
• To help with something else (for example, checking whether the club will hit a branch on the backswing): you may stand there, but only if you move away before the stroke and it isn't part of a regular routine.
• There's no penalty if you were standing there inadvertently.

Before the player starts taking their stance, you may stand behind them and help with aiming.

Penalty: two strokes for your player in stroke play (loss of hole in match play).

Source: Rules of Golf (2023), Rule 10.2b(4).