When the Virtual Steem behind Steem Dollars represent 10% of the Virtual Market Cap
Before Hard Fork 20, Liquid Rewards if the total Virtual Steem locked behind SBD represented more than 2% of the Virtual Market Cap, liquid payouts would be made of a mix of Steem. After 5%, payouts would be entirely Steem.
After Hard Fork 20, liquid rewards pay out fully in SBD until 9%. Between 9 and 10% it is a mix, and beyond 10% payouts are Steem fully.