How do you keep the silent part of a video?
Welcome to video Stack Exchange. This is an interesting question, and one that I have asked myself in the past!


If you are willing to spend money, check out the third-party Adobe plugin Awkward Pause (click here for demo video).
For a quick and dirty solution, you can try using this command-line tool I made last semester for a class. It is different from carykh's in the practical sense that it does not need to store png sequences of the images. (Therefore it requires less space.) 


For your purposes: keep the first_speed = 1.0 and change the second_speed to a very high number, then compile in XCode.