The most relevant part seems to be on page 11:Rebalancing...

The most relevant part seems to be on page 11:

Rebalancing announcements are made on the first Friday of March, June, September, and December;
therefore, a two-week notice period is provided before the rebalancing takes effect at each review. p.11


So it is the two week period after the announcement, the nearest example being this Friday, that Funds that have self governing rules that require a representative holding of all ASX 100 stocks have to comply. That being said, these Funds have teams of professionals that can predict the likely new inclusions and may choose to get ahead of the game I assume. So it might be buying in the two week window, it might be cross-trading shares already held into the right 'bucket', they might not need to strictly comply with the 'two-week' part...who knows? Point is, at its most basic, that shares have to be bought at some point.