Sag defense is called before pulling. This defense is ONLY run until the disc gets into a CUTTER'S HANDS. Keep in mind, this is still a variation of man defense.
The mark sets up in a straight up stance. The defenders that end up guarding handlers will "sag" off and guard the cutting lanes (10 yards off). Defenders guarding cutters stay just slightly under the cutter they're guarding.`
If the disc is swung to a sideline, the sagging defender picks up the handler and forces towards the middle of the field. The old mark sags off and guards the lane. The far far dump defender stays near his man.
If the disc is swung back to the middle, everyone sets up the same way, marking straight up and sagging off the dumps.
Here's the same setup when the disc is swung to the other sideline.
When the disc finds itself in a cutter's hands (thrown up field), everyone falls into a regular man defense.