It looks to me like what this app does is repackage (known as remuxing) the data streams inside the MKV container, rather than actually converting anything (hence the fast operation). Because most MKVs you get from the net contain H.264 video and AAC and/or AC3 audio, the video and audio streams can be passed through to an MP4 container with no actual conversion necessary (I think that text subtitle streams can also be passed through, but VOBSUB streams don't work in MP4). This process is very quick and makes the video compatible with a wider range of devices, including iTunes, PS3, etc. However, Subler will do the same thing for free, so I'd check it out before shelling out for this.

What's up with that screenshot? And just for the record, "mkv" is a *container* format, not an A/V codec like xvid, x264 or divx. The description is too vague for me to get much of an idea what this app really does.