Underbetting and Overbetting: Theory
As people were learning HUSNGs, a general consensus arose that bets should tend to be between half the size of the pot and the full size of the pot. That works as a good rule approximation for most situations, and keeps your moron roommate from dumping too much at the $2 games during an ill-advised microstakes backing agreement. However, there's nothing mathematically magical about that half pot number, or that full pot number. In fact, there are plenty of situations where betting smaller or bigger is theoretically much better. The reason for this is not derived from gimmicks or “soulreading” by making your opponent guess wrong. It comes from something somewhat less sexy – the math.
Here’s what an overbet and an underbet says to your opponent, when used appropriately:
Underbetting: My range is stronger than yours, but I have plenty of complete air, too. Your range is full of total garbage, and we both know it. I’m going to bet small: What are you going to do about it?
Overbetting: My range is stronger than yours, but I have plenty of bluffs, too. Your range is full of mediocre bluffcatchers, and we both know it. I’m going to bet big: What are you going to do about it?
The key difference is the composition of your opponent’s range. It is weak in both cases, but in one, the weakness consists of total air hands that will need to be folded even to a small bet. In the other, the weakness consists of plenty of mediocre hands that can call small bets, but are forced to make massive hero calls against big bets. Our range remains fairly consistent: More monsters than our opponent, but also a good amount of air that is looking to steal the pot as effectively as possible.
We’ll go more into detail about underbetting and overbetting HUSNG examples in the next two articles, but it’s worth noting that this characterization is widely applicable throughout most poker games. In MTTs, we often make small 3-bets (and 4bets, and 5bets…) because it represents a strong range, our opponent has plenty of weaker hands that still have to fold despite the size, and it doesn't really matter that we too can have plenty of garbage hands. “What are you going to do about it?” is exactly what we're asking. That's also why open raise sizes in MTTs went from being 3x or even 3.5x down to 2-2.2x as the game evolved. “I'm opening the pot. I have a stronger range than you do. What are you going to do about it?” The smaller size allows you to contest more pots and risk less while doing so.
As for overbetting, you probably know that feeling of getting ready to call a bet with a marginal holding, and raising your arms in frustration as a much bigger bet comes your way, having no idea what to do now. The bigger the size of the bet, the more often the bluffcatcher needs to be good for his call to be correct, which means that the bettor can include more and more bluffs and win the pot with air more and more often. An overbet from a player with both monsters and air in his range, when you rarely have a very strong holding, is a nightmare for even the best of players, as you can see by the ribbing that Tom Dwan will occasionally get for developing a reputation of getting abused by bets larger than the size of the pot.
Overbetting and underbetting have strong foundations in unexploitability in these situations, but that doesn’t mean you have to be balanced – most of the time, you don’t want to be. You want to be using your bet sizings to further exploit your opponent and get the result you want from the hand you have at the time. Sometimes you underbet only when you have a bluff, because your opponent just does not have the stones to raise without a hand himself, and you might as well get full value when you have it and get off as cheaply as possible when you don’t. Sometimes you underbet only when you have a value hand, because your opponent just can’t help himself and will be constantly induced, so you make a small bet for value when you have it, and advertise a more “normal” bet when you don’t. The same goes for overbetting. Some opponents love to make those hero calls with bluffcatchers, and some wouldn’t dream of it. Don’t worry much about unexploitability against opponents who are just begging to get exploited with their approach to the game.