It adds 1d6 to the effective level of the cleric to determine what creatures can be turned and 1d4 to the number of creatures turned. After extensive testing, I have been unable to notice a difference in effective turning level between a cleric with and a cleric without the sun domain.
My efforts at trying to test if the extra 1d4 creatures is working has proven to be an exercise in futility. Ever tried explaining to someone that you have gathered the 20 or so undead creatures in a room for testing purposes and could they please not kill any.
On a side note, considering turn undead used to cause fear for 10 rounds or outright destruction if you were high enough level, the 1 round knockdown seems a bit low. I have not figured out how the damage is calculated but considering the undead you damage are the undead you would normally destroy it would want to be a substantial amount. Perhaps the knockdown duration could be increased to 1d4 rounds + charisma modifier? As for the damage, I have no opinion on that yet seeing as I do not know how the damage will scale up in the higher levels.

