The second Stud betting round, once each player has 2 up cards.
From here on, the player with the strongest visible board acts first (in Razz, the best low). Bets stay at the small bet. In Stud, if a pair shows on the board, the player may choose the big bet (open pair). In Razz, from a 3-card low you catch a good card 41% of the time and a brick 59%, and whether you fold here is the most important decision.