In most videos, there'll be a whistle-blow that marks the point where the life guard in the video reacts to the drowning person. Upon clicking the right location, you are "graded" based on how quickly relative to the life guard you reacted. You get the "answer" by looking at who is getting rescued.

In my case, it started with a video that appears to be broken (?) the video just ended without any life guard reaction, but you can hit "Play Another Video" which appears afterwards.