Rule 13.7c: -
For an offence during the taking of a penalty corner : a defender, other than the goalkeeper, crosses the back-line or goal-line before permitted, the offending player is required to go beyond the centre-line and cannot be replaced by another defender : the penalty corner is taken again.
If a defender at this or any subsequently re-taken penalty corner crosses the back-line or goal-line before permitted, the offending player is also required to go beyond the centre-line and cannot be replaced.
A penalty corner is considered as re-taken until any of the conditions of Rules 13.5 (for PCs in normal time) and 13.6 (for PCs at half-time and full-time) for its completion are met.
A subsequently awarded penalty corner, as opposed to a re-taken penalty corner, may be defended by up to five players.
If a defender crosses the centre-line before permitted, the penalty corner is taken again.
The Umpire ensures that both teams are ready and advises that the Penalty Corner may proceed. A defender crosses the goal-line before permitted. The Umpire then 'holds' the Penalty Corner, while the defender takes off his protective equipment. He also explains to the defence that he had indicated that the Penalty Corner could proceed. The breaking player runs to beyond the centre-line.
The attachment shows the relevant slides from the 2015 Umpire Managers Briefing for Umpires.