Offense sets up with three handlers back and a horizontal stack. As a defense, this is what we want.
D1 is putting on a hard force and not allowing any break throws, especially up field. D2 is taking away the easy open-side dump throw to O3.
D3, D4, and D5 make up the "wall." They edge towards the open side and take away easy up-line throws. D4 and D5 'faceguard' the offensive players. D3 protects against the shallow break throw, and is also ready to poach any strike cuts coming from O1.
D6 and D7 are working together to take away deep throws and break throws. Whoever is on the break side (in this case, D6) pinches in to take away break throws to O4 and O5.
If the disc gets swung to O1...
The defense rotates to take away the threats. It is important for the chasers (D1 and D2) to react quickly and use their speed to put on a hard force. With more of the field being on the open side, the deeps need to be more concerned about hucks.
If the disc gets swung to O3...
Our base defense forces middle. This means D2 needs to hustle to put on a backhand force on O3. D1 takes away the easy swing to O2. The wall and deeps make the necessary adjustments.