Early round investors would be diluted by subsequent rounds, so they protect against that by putting the pro-rata provision in the contract that lets them inves

Early round investors would be diluted by subsequent rounds, so they protect against that by putting the pro-rata provision in the contract that lets them invest in later rounds (so that they can reverse the dilution). In the past YC decided not to exercise this right because if they did so selectively, it could be used to indicate what YC thinks about a company, which is mostly bad for those that didn't get re-investment. The change is that YC is now going to have an objective, public criteria for exercising this right, so it can't be used as a signal, but YC partners/investors can get the benefit of the rights they negotiated for.