I am working on product in which I am a newbie, but in my group there are coworkers that know that product deeply in all the details. Those coworkers are busy with high-priority tickets and I am new to the product and I am learning it. I have own small knowledge area in which I am learning the product step-by-step.
I see the need of creating tools for speeding up the development process and I have opportunity to create such tools and learn more about the product. I have often calls with one coworker and he explains me the product but he is very involved to that business area and project and he explains me all that stuff very very deeply, so I don't know many things that he tells me (I am not on his level). I am asking him for short simple questions, but every time he starts explaining one question, he goes very deeply into the topic and it is hard to say stop.
This is quite awkward because I forget 80% of knowledge that he is saying and also my manager told me to not abuse him but let him do his work. We have usually long 3/4 hour calls and he tells me everything about that product and business area. This is problematic also because we work in different time zones, I work from EU and he works from USA. So we should have 4 working hours in common, but sometimes I work 10 hours a day when I have call with him.
So my question is how to tell him to shortly describe things I need to know for my "small" knowledge level, in a polite way, as to be not understood like "I am not interested in what he is explaining to me".