Hawthorn have played a secret scratch match at Etihad Stadium so the AFL could trial potential rule changes.
The league invited the Hawks to hold the training session at the venue on Saturday over their bye weekend and the scratch match also involved AFL umpires.
It is rare for clubs to train at Etihad Stadium and the session was only revealed on Tuesday.
Hawthorn usually train at their Waverley Park headquarters, which is open to the public.
The AFL football operations manager Steve Hocking was among top league officials who watched the session, which was filmed for more analysis.
Congestion is a big concern in the game and one solution being touted is starting positions for players at centre bounces.
One of the experiments during Saturday's scratch match was having three or four players from either team starting inside the 50m arc at centre bounces.
League officials also used AFL players to experiment with the rules during the pre-season and there could more trials before the end of the season.
Hocking has indicated that any rule changes for next season will most likely be announced in early October.