So, most of the time when your opponent shoves a big bet in the pot - they have a good hand.
But why couldn't you have a tight player that bluffs only 5% of the time? 1 in 20?
Not sure where Harrington got the 10% number. Seems too rigid.
I am pretty sure I first came across this in his tournament books. You can read most of his logic and understanding there. The main point is that people, even conservative players, know they need to bluff and do. The probability that they could be bluffing in a big spot should always be considered at least 10%. This is because there is a tendency for people to not assign bluffing ranges. Against certain players, this chance is almost certainly higher. It should not be lower. There is an assumption that your opponent plays reasonably well. When you are considering the range of
hands an opponent would make a big bet with, and your
equity against that range, the 10% bluffs will basically give you a little bit of an equity boost that can shift you from a fold to a call, if it's close. Really, if it's that close that 5% bluffs would be a fold and 10% bluffs is a call, then you could probably flip a coin to make the decision.
Now, I am primarily a cash game person. In those games, the 10% minimum rule is almost certainly important to remember. There are spots where people do not bluff nearly enough. But, even in most of those spots there are some bluffs. Against good players, there will be more than 10% bluffs in most spots. The reason for this is because then you can always fold to his big bets and he will never get paid off on the big bets, if they aren't bluffing. The better you and your opponents are, the more you need to get to the optimal bluffing amount for your bets which is usually higher than 10%. If you're betting 1/2 pot on the end, you need to be bluffing 25% of the time to make it so your opponent can't just fold all the time profitably,
In most games people aren't not bluffing nearly enough, or they are bluffing way too much. You just need to watch what they are doing and use that to make your decision. In a lot of spots, 10% chance of it being a bluff doesn't change the fact that you're folding.
Anyway, 10% is really, really low for the
odds someone is bluffing. You need to draw a line somewhere for a minimum and that's as good a spot as any.