But how do I get But How?

To the best of my knowledge, it's achievable in the same way it was in Payday: The Heist, and you can find that information in this question.

Typically, if you don't do anything, the truck will fall bottom-first into the slaughterhouse and you will need to C4 its doors open; however, you need it to fall nose-first into slaughterhouse so you would need to shoot the wires to get the truck down (of course, you don't want to shoot the wires!)

1. Once the map starts, head to the waypoint:

_{Waiting at the start of the map for the convoy to appear down the ally}

2. The convoy with the truck will come down that ally. When you see the red armoured GENSEC truck (third in line) come, shoot out its front window. This is vital, and will ensure the truck lands nose-first. With its window shot out, you will see it gets hit differently:

_{The truck being hit with its window shot out}

If you do not shoot the truck's window, it will be pushed like this instead:

_{The truck being hit without its window shot out}

3. With the done, proceed as normal into the slaughterhouse, and you will notice that the truck is hanging with its nose-down:

_{Truck hanging nose first after its window was shot out. Notice that the objective is slightly different, as you need to get the truck down to continue the mission}

However, if you don't shoot the truck window you can see the truck hangs with its bottom down (and you can just plant the C4 to blow it open, rather than needing to get it down at all!)

_{The truck hanging normally if the user didn't interfere}

4. With the window shot and the truck hanging nose-down, you can see that it's possible to shoot the wires now, but you don't want to shoot the wires! After several minutes, or about once the second wave has ended, the truck will fall on its own and you will get the achievement!