I use Impress a lot for demonstrating the Eclipse IDE (Java programming). During my presentations I switch between the Impress and Eclipse application frequently. Sometimes I end up in the Impress application and not in the running presentation (which is in full screen mode). Unfortunately at this moment F5 is inactive and I have to use the task manager to find the presentation. I think it would be a very useful feature if in such a situation F5 could switch to existing running presentation.
Thank you for reporting this request. Seems like a usefull feature to me.