Ah, thank you for explaining the connection between A and Z!
Can you please also explain Q? To reiterate the key point from my earlier (rather lengthy) email:
How exactly does Q integrate with Z to provide X? And how many P can I expect from Q because of Z?
There's nothing wrong with simply reiterating your question. Just be sure to acknowledge the information they gave you, even if it wasn't what you wanted.
Acknowledgement is the key. When you acknowledge first, they will not be upset or offended.
Even if you can't make head or tail of the response, you can still acknowledge the fact that they responded. For example:
Hi B,
Thanks for your rapid response. I'm sorry, but I don't really understand what you're telling me about F. Is that connected in some way to Q?
What I would like to know is how Q works. More exactly, my question is:
How exactly does Q integrate with Z to provide X? And how many P can I expect from Q because of Z?
Just politely repeat the question. It actually works better if you repeat the exact question than anything else, providing of course that you word your question clearly and succinctly in the first place.
(I know this is an unusual suggestion to repeat the exact question rather than changing it up; all I have to say is: try it. You're likely to be surprised. It may seem counter-intuitive, but I have observed the results carefully and I speak from experience, not intuition.)