So uh, I did a bit of digging (i.e. decompilation, coupled with confusing the hell out of myself) and discovered the code. Stand in the same area as Trixie and type "summerfun".
To be honest, I'm a bit underwhelmed. It does have a few nice QoL features (medikit/journal effectiveness booster, experience boost, and a toggle that resets your sanity to 100 every turn) but aside from that everything it does can easily be done via the debug commands.
I'm also not sure how often the code gets rotated, so I have no idea how long this is going to be valid.