Today:
2nd barrels
1. Why
2. When
3. Against whom
4. Against whom not
5. Sample hands
1. Why?
As always there are 2 scenarios:
a) for value - we're ahead of the opponents range and expect to get called from worse hands.
b) as a bluff - we're behind against the opponents range, but we expect him to fold enough hands that beat us.
2. When?
(i) Value
We have a strong hand and want to get value. There's not too much to discuss here except:
sometimes value not as obv, can valuebet thinly vs draws and medium strength made hands, especially when opponent bluffcatch-happy
(ii) Thin value
We have a medium strength hand and want some more value but check/calling would be non-optimal because the opponent would check back worse valuehands and draws.
This is a bit more of an interesting spot already.
[See sample hand 1]
(iii) Bluff with a lot of foldequity
When scarecards fall on the turn that hit our perceived range (the range the opponents expects us to have).
In general these scarecards are overcards to the board or for example a Q on a Khigh board.
[See sample hand 2]
Another opportunity for bluff2ndbarreling arises when a lot of draws get there that we can represent.
[See sample hand 3]
(iv) Semibluff with decent foldequity + decent potequity
This is also a common spot: we raised a hand preflop, bet on the flop and the opponent called. Now on the turn we pick up a card that gives us a lot of additional equity - we can not bet solely with the potequity, but sometimes there are some hands in the opponents range we can get to fold.
[See sample hand 4]
Note 1) The first thing to consider is always the opponents range.
Note 2) Before betting also consider your range! Are those draws that just completed even possible for you to represent?
Note 3) Consider your image!
3. Against whom?
Once more: The opponents range is the first thing to consider!
This is read-dependant. In general everybody who calls a lot of cbets is a good target because his turnrange can not include many strong hands.
This applies for TAGs and rocks as well as loose-passive and maniac-like players. However, there are a few exceptions (see below).
Once more: reads are the key to successful 2nd-barreling! Just mindlessly firing scarecards on the turn is burning money against most player types.
Actually recreational (i.e. loose/passive) players can be good candidates for 2ndbarreling because they call a lot on the flop but fold a lot on the turn (keyword: reads!).
4. Against whom not to 2nd-barrel?
Some players call a lot of cbets and therefore have a very weak range on the turn - but they still refuse to fold, may it be by calling again or by raising.
Obviously those are the ones you should either not 2nd barrel or fire 3 barrels against (which I would only recommend if you're very sure that what you do is correct).
The people to watch out for are (generally speaking) rocks and maniacs.
It's dangerous to be aggressive against a rock because he has a hand most often.
Note1: A rock might call a lot of cbets, but he still has a strong range.
Note2: A maniac might not call a lot of cbets but he does have a weak range when he calls because he raises all the strong hands.
5. Sample Hands
Hand 1:
NL Holdem 0.5BB/1BB
SB (100BB)
BB (100BB)
UTG (100BB)
MP (100BB)
CO (100BB)
Hero (100BB)
Dealt to Hero K♦9:s
fold, fold, fold, Hero raises to 3BB, fold, BB calls
Flop (7BB) T♣9♣4♠
BB checks, Hero bets 5BB, BB calls
Turn (17BB) 5♥
BB checks, Hero ???
-> Hero bets 12BB
9BB can be OK too.
Note: Keep balancing in mind!
Hand 2:
NL Holdem 0.5BB/1BB
SB (100BB)
BB (100BB)
UTG (100BB)
MP (100BB)
CO (100BB)
Hero (100BB)
Dealt to Hero xx
fold, fold, fold, Hero raises to 3BB, fold, BB calls
Flop (7BB) Q♣9♣4♠
BB checks, Hero bets 5BB, BB calls
Turn (17BB) A♥
BB checks, Hero ???
-> Hero bets 14BB
Note: This is an opponent-dependant spot.
Hand 3:
NL Holdem 0.5BB/1BB
SB (100BB)
Hero (100BB)
UTG (100BB)
MP (100BB)
CO (100BB)
BU (100BB)
Dealt to Hero xx
fold, fold, fold, BU raises to 3BB, fold, Hero calls
Flop (7BB) 7 :diamond:6 :heart:3 
Hero checks, BU bets 4BB, Hero raises to 16BB, BU calls
Turn (39BB) T♥
Hero ???
-> Hero bets 31BB
Note: Once more this is opponent dependant (he needs to be capable of folding an overpair).
Hand 4:
NL Holdem 0.5BB/1BB
SB (100BB)
BB (100BB)
UTG (100BB)
MP (100BB)
CO (100BB)
Hero (100BB)
Dealt to Hero A :diamond:5 
fold, fold, fold, Hero raises to 3BB, fold, BB calls
Flop (7BB) Q♣9♣4 
BB checks, Hero bets 5BB, BB calls
Turn (17BB) 3 
BB checks, Hero ???
-> Hero bets 12BB-14BB