I took lessons at Bayhill a few years back and the club pro, who is friends with Mark O'Meara, explained Tiger's practice putting ritual.
He begins by taking a spot 3 feet from the cup and putts until he gets three in a row. Then he does it from another spot, 90 degrees up, and still 3 feet out. Again, he must make three in a row. If he does, he moves on to a point 90 degress from that. Basically, he is putting from the same distance of 3 feet, from four different angles to the hole.
Should he miss anywhere along the way, he forces himself back to the original spot for the distance involved and starts that level over. He does it at 3 feet, 5 feet, and 8 feet. Statistics show that the average birdie putt for PGA is 8-9 feet.
It will sometimes take him up to two hours to complete the drill.