well, really just force them down. Like a chin-up or lat pulldown. It's an actual force thing. If your back is tight enough (shoulders pulled BACK and together), the shelf should still be there when the elbows drop. The looser the back is the less you can drop the elbows because you are using them to create a shelf to set the bar on. So the fear is that you'll lose the bar off your back right? Well, if you tighten and squeeze your back together, and arch hard like you're doing a barbell row held to your chest, then it becomes "easier" to keep the shelf with elbows down.
Back tight means:
1) shoulders drawn back, not just your arms, your actual shoulder blades
2) think "stretch the chest" to pull those shoulders back
3) arch hard like holding a barbell row to your chest or an RDL
4) squeeze that as tight as you can and keep it there lol.
This is one reason I like training the back isolation work with a "squeeze and hold" contraction at the top. It's super draining, but it teaches you to hold that position, which is exactly what you need to do in a squat and powerlifting style bench.