This video shows the same clip three times, each managing the frame rate in different ways. Set KZfaq to play this video at 60fps, or else both of the clips on the top will look identical.
The top-left clip is the original clip recorded at 60fps. The animation is noticeably smoother than either of the 30fps versions. This is the preferred format for video uploads, and it's somewhat shameful it took KZfaq so long to support it.
The top-right clip is reduced to 30fps by simply deleting every other frame. Most frame rate conversion is done this way, including HD KZfaq clips being played at non-HD resolutions. Since literally half of the animation information is lost, some of the smoothness is lost and it looks kinda terrible next to the 60fps clip.
The bottom-left clip is reduced to 30fps by creating new frames consisting of blending pairs of adjacent frames from the source clip together. Although individual frame stills are no longer accurate, more of the animation information is retained and the clip appears smoother as a result (but not as smooth as genuine 60fps).