Remux: the conversion that isn't one
Most MKV files carry H.264 or H.265 video — exactly what MP4 holds too. So instead of decoding and re-compressing two gigabytes of movie, this tool simply repacks the untouched streams into an MP4 container. That is a remux: it finishes in seconds regardless of length, and the result is bit-identical to the source, because not a single frame was re-encoded.
Converter sites that make you upload the file first turn this seconds-long operation into half an hour of transfer. Here the whole thing happens on your machine — no upload, no queue, no watermark.
And when the codecs say no
Some MKVs hold streams MP4 cannot carry — exotic video codecs, certain audio formats. The tool detects this and falls back to a proper re-encode automatically: slower, and lossy in the way any encode is, but it produces a file that actually plays. You get the fast path whenever it exists and the honest fallback when it does not.