https://bowlroll.net/file/60450 has a description, that translates to this:
"This is a tool to fetch motions in the vmd format from MikuMikuDance's pmm files. In addition to that, it fetches accessories, parent settings, and so on. It's intended to be used for periodic motion backups."
And here is my English translation of this blog entry: http://tracebox.blog.fc2.com/blog-entry-36.html . (Note: All text in brackets was added by me, not written by Mr. Tracebox).
PmmDump 2 motion fetcher:
Happy New Year (2015), even if it's a bit late. My best wishes for this year too.
At the end of last year I was making a Christmas video, but I got into the situation that most of the about 10 pmm files for my video could not be opened (by MMD).
A model I was using had the 'nakaguro' black dot character （・中黒・） included in her name. Nakaguro's internal character code in MMD is cp932, "[What? The character format of PMX is UTF, right?]" and such suspicious reasons seemed to exist (for me), but I realised again the importance of backing up motion data (as vmd files). [Even if you back up your pmm files, it's not meaningful if MMD cannot open them.]
About PmmDump (1), that stopped being usable when the new version of the pmm format came, I have updated it for use with the new version.
I have been dissatisfied for a long time, that after replacing the model, about external parent and accessory parent settings, the hassle of having to start over with those, I hope I can resolve this eventually, but the necessary information is even written out. (Probably means: Please use the information written out by PmmDump 2 to get around this hassle for now).
Its output log is still sloppy, but I think I will brush up (PmmDump 2) while I'm using it. For the time being, I'm uploading it as it is.
(Note about versions: MMD 9 users could not use PmmDump 1, but MMD 9 users can use PmmDump 2, most likely.)
So, if MMD is unable to open your pmm file with your precious new motion that you have made yourself, then you can apparently get around this by letting PmmDump 2 fetch your new motion from your corrupted pmm file as a vmd file? If so, you could start a new empty MMD project, load your model, and then finally load your salvaged vmd file onto your model.
Mr. Tracebox writes in his BowRoll description that you are supposed to use PmmDump 2 for backup before the disaster strikes. But he hints in his FC2 blog that he used PmmDump 2 himself to salvage the motions after the disaster struck.