How to prepare for questions about the part of the paper that you didn't write? I'm going to present a paper, in which we propose several algorithms to solve a problem. One of the algorithms is actually developed by co-authors at a different university, who will not attend the conference.

I only understand this algorithm at a very high level, and I have no idea how it is implemented, which parameters was chosen etc. It uses some techniques that I have very little background. It would take me months to understand all the techniques and the implementation. And to be honest, I'm not really interested in this algorithm since we have other alternatives that perform equally well if not much better.

However, if I am asked about this algorithm at the conference, how I should reply. I couldn't say somebody else did it, and I don't know.
couldn't say somebody else did it, and I don't know.


Yes, that is more or less what you should say.
It is completely normal for collaborative works that different co-authors are knowledgeable about different parts of the work.
In particular, works that span several otherwise separate areas, it is not unusual for experts to contribute their respective techniques to create an integrated solution together.
In such a case, the various co-authors basically see the components contributed by others as some kind of a blackbox. They should know what those components do, but they do not necessarily know how they do it.

In such a case, it is reasonable to answer:


  The ... algorithm is a work by my co-author ... who unfortunately cannot be here today. I recommend to direct specific questions on the algorithm to them.


You should be ready to provide some way of contacting the co-author (e.g. e-mail address on the slides).