Here is a basic vertical stack setup. Cuts come from the back, and the mark is forcing to one sideline. There are several types of poaching. I will demonstrate four of them here.
The "first cut" poach
The front-of-stack poach
The handler poach
The basic "Bracket" poach
The "First cut" poach is designed to help the defender more easily predict the movement of the first cutter. They play out in the open space, making both the in and out cut more difficult.
The most common way to counter this poach is to make a cut to the break side of the field. If the player with the disc is a capable break thrower, this should be the obvious choice.
If not, or if the weather is particularly bad for that kind of throw, an easier way to complete that move is to continue the break cut all the way to the reset position. Communicate to the player currently occupying the position to make a reset move, clearing the space for the cutter. If the reset doesn't get the disc, the cutter will be very open for an additional reset.
The front of stack poach is another common defensive maneuver. Here, the defender nearest the disc attempts to stop all up field throws to the open side, regardless of who is cutting.
Much like the last situation, it would appear that a simple break throw would be easiest to complete; however, the limited space between the disc and the cutter limits the amount of space and time needed to make quick reactions to the throw and cut, making it an opportunity with a greater chance for error.
Instead, the poached cutter should "challenge" their defender, making a strong, fast movement straight at the space the defender is guarding.
This will do two things: First, it will take the defender's attention off the disc and the area behind them, making it easier for the rest of the offense to function. Second, it will create an open cutting space directly in front of the disc, which the cutter or reset can use to quickly move the disc upfield.