Here is a fairly common configuration we will find ourselves in when playing 2-4-1. While we are "set" (ie the disc is static and our defensive pieces are in-position), Blue5 and Blue6 have relatively simple jobs: front your person while staying close, and squeeze the field (pinch towards the middle) when there is no attacker in your space. Always defer to the DeepDeep's voice.
The disc will move and the angles will change. The force may even change, as it does here: Blue5 and Blue6 make minor adjustments while staying close. Pay particular attention to Blue5: due to the change in force, they can play inside of Red7 instead of in-front of Red7. Blue5 is beginning to squeeze the field while staying close enough to stop an upfield throw to Red7 if Red1 were to get the disc on the breakside. Blue4 would be able to stop the break-continue to Red5 - so don't get too sucked-in!
The disc gets to the sideline, let's say that Blue2's mark is broken in this case, and Red1 is running onto the disc in space. As a team, we have only one goal in this moment: to stop upfield continues. The wing and mid on that side are responsible for stopping the first look, and will do so by moving in the indicated directions.
The next throw is contained, buying chasers time to get in position; we already do this very well. Now that we've achieved our primary goal (contain), we can look to create a defensive opportunity.
The marking chaser (Blue2 in this case) ALWAYS WANTS TO MARK STRAIGHT-UP WHEN NEAR THE SIDELINE. This is critical. When teams sense a force middle zone, they try to swing it to the sides. We use the sideline as an additional defender because we anticipate this response - even though the zone is force-middle, it is most suffocating near the sidelines.
In this set position, we see present the thrower with a large throwing wedge in the midfield, but we have our ShortDeep and Beast already in this same space. Anything in that space is contested. Decent throwers will not try to force this, instead, we want to invite and cover the following dump cut...
Red3 will come behind the thrower to get the disc. Blue1 must try to stop this, but should not over-pursue to the right of the disc - offense gains no advantage by getting a reset even closer to the sideline.
The most viable throwing lane/wedge at this moment is now highlighted. If we can get here, we have succeeded as a defense: Red1's best look is a long, risky swing, that even if completed does not break the zone.
Blue3 is aware of this target space, and will be ready to make a play in the lane. The far wing, Blue6, squeezes in and towards the handlers, helping Blue3 and also moving into position to make a play on a long-swing. NOTE: whenever the disc is near a sideline, the OPPOSITE WING (now Blue6) SHOULD PINCH IN, and LET THE MIDS KNOW WHERE THEY ARE.
Good job team! Now they're probably going to try to force something into one of these three areas, to R2, R3, or R4. R2 is really the only person who could receive in more than one of these zones, so every other attacker should be relatively easy to anticipate. Otherwise, expect a punt.