I can say that I absolutely did not work it out before performing it, back when I was a DMC. I had no idea who my buddy was going to be, so any 'working it out' in advance would have been of little value. I was handed the exercise on an evening in the pool, with another DMC, from a different class, who - like me - needed to finish skills scoring. As it happened, we both needed to complete Exercise #5, and the Instructor asked if we wanted to try it. My buddy was of a substantially different size than I (shorter and lighter). While he and I were friends, we had never trained together, had actually never been dive buddies, and we had no anticipation of doing the exercise on the evening we did it. So, we did what we were supposed to do - used the 5 minutes for planning, and did the exercise. It was a blast. We had to work some things out - aka problem solve - as we progressed, getting into a good buddy breathing rhythm being one example. OK, it helped me, a lot. After the completion of the exercise was really the first time that I actually felt confident that I could handle almost anything, IF I kept my wits about me and 'worked the problem'. Frankly, up until that point, buddy breathing was something that caused some real anxiety, as just one example.
Many of the DMs I have certified have said the same thing - that it helped build their confidence. And, in all cases, none of them have known, in advance, who they would be buddied with for the exercise. Sure, they can read the requirements in their IM, they know it is coming - sometime - so, yes, they can think about it in advance. . But, when they are given the problem, while in the water, with 5 minutes to plan, it requires organization, thought, compromise. As an Instructor I really enjoy seeing how DMCs work it out. In one case a number of years ago, I had a DMC who was, at best, 'OK'. He was a teenager, and much younger than his four classmates. And, although he had met all the performance requirements, he hadn't exactly shown the kind of maturity and focus that I preferred in a DM candidate. I was frankly on the fence with regard to certification. But, in Exercise #5, he really blossomed. He and his buddy performed what I have since described as a 'demonstration quality' example of the exercise. And, he came out of the pool with a smile on his face that lit the deck! It changed my perception of him substantially.
If that hasn't been your experience, so be it. We may all differ in our reactions to experiences. Nothing right/wrong, good/bad, about that, it is just human nature.