Definition of narrow the field
If you are the type to raise preflop to 3bb, your iso-raise might be 3bb + 1bb per slider. In other words, to use our example above, 2 players have limped so if the button makes an iso-raise, it will be 3bb + 2bb so he will raise to 5 big blind. He could have made it bigger if he was out of position. The advantage of the iso-raise is that it will narrow the field. The more players in the hand, the lower our equity will be. By making an iso-raise (bigger than a standard raise), either everyone folds and we win the blinds, or we end up (most of the time) heads-up in position with the initiative (advantageous position).
The advantages of narrow (the field)
