Rules 13.7d: -
For an offence during the taking of a penalty corner : a goalkeeper, or player with goalkeeping privileges, crosses the goal-line before permitted, the defending team defends the penalty corner with one fewer player : the penalty corner is taken again
The defending team nominates which player is required to go beyond the centre-line, and they cannot be replaced by another defender.
If a goalkeeper, or player with goalkeeping privileges, at this or any subsequently re-taken penalty corner crosses the goal-line before permitted, the defending team is required to nominate a further player to go beyond the centre- line, and they 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.
The Umpire indicates that the Penalty Corner may proceed. Prior to the ball being injected, the goalkeeper crosses the goal-line before permitted. The Umpire then 'holds' the Penalty Corner, advising that one of the defenders has to go beyond the centre-line in place of the goalkeeper. The player nominated by the defending team then runs to beyond the centre-line.
The attachment shows the relevant slides from the 2015 Umpire Managers Briefing for Umpires.