You need to be able to account for your time on your various visas when you exit the common area at Belarus. You would hold on to your boarding pass to show when you arrived in Belarus, and show it on exit. You will also surrender your Russian migration card showing the dates you were in Russia. The Belarussian authorities then match up the dates of your visas with the times that you were in the two countries.

That's my understanding, anyway, from planning a trip i ultimately didn't take.