number one on the side and don't touch the back, number six on the top and don't cut it wack, Jack.
– Mullet Head, Beastie Boys (1994)
Before the Beastie Boys, Compaq embraced the mullet. In 1982, they reverse engineered the BIOS of the IBM PC to create compatible and quality IBM PC clones. It was IBM in the front, Compaq in the back. Mullets are coming back to software (in reality, they've been here the whole time).
The way that mullets work in software is taking a well-known and trodden API (the front) and doing something wildly different in the back (that still fulfills the same contract).
Taking traditional FinTech APIs for things like processing payments and implementing them with alternative pipes, like crypto.
Windows implementing the Linux kernel interface on top of the Windows kernel in Windows Subsystem for Linux
AWS offering Redis/MongoDB/Elasticsearch-compatible APIs powered by proprietary tech or different open source projects
Mullets let you draft off of a successful product's distribution while doing the job faster, cheaper, or better. Whatever gets the job done.