Under-raise post flop

[vinnie]

Hi,
i came across this situation the other day and i would like an officail ruling on this particular situation.
i believe i knew what the answer was but i got told a different answer.

Blinds 100/200
Flop:
SB: checks
MP: bets 500
LP: calls 500
button: all-in 625

now the question is what are the options for small blind?
are his options open and can re-raise to whatevr, or by checking that is an "action" and can only call the 625 all-in?

thanks

[HalfSugar]

He has all options open to him because although he has already acted in this betting round, the action subsequently changed when the MP player bet.

The action for MP and LP now rest on what the SB does. If he calls the 625, they can only call or fold. If he raises to 1,000 or more, their options are wide open again as the action will have changed since they acted. An under-raise does not count as changing the action in a hand.

As an aside, had the SB been the bettor of 500 and the MP and LP called before the BTN under-raised, the SB would only be able to call or fold, as would the MP and LP players.

[vinnie]

thanks for the reply.
i agreed with that completely. but is there anywhere in the rules to back this particular scenario up because my friend showed me a rule to prove what you just wrote wrong.

"All bets must be equal to or greater than the size of the previous bet or raise on that betting round, except for an all-in wager. A player who has already checked or called may not subsequently raise an all-in bet that is less than the full size of the last bet or raise. "

so now i dont know where i stand. hahaha

[HalfSugar]

There's no way that can be right. Can you find the source?

Let's say an EP player checks, if a MP player bets that re-opens the EP's action and I cannot possibly imagine that a LP all-in under-raise can close it again.

"...A player who has already checked or called may not subsequently raise an all-in bet that is less than the full size of the last bet or raise. "

This bit is vague but it must mean when the action goes:

Check, check, under-bet all-in, back to first player
or
Bet, call, under-bet all-in, back to first player

In both scenarios, no action is open. In your specific case, the action changed when the MP player bet out after the early check.