I just googled this, do you mean https://softwarebakery.com/projects/drivedroid?
Sounds rather nice, especially considering I've been installing various systems over the last few days and flashing images to a thumb drive gets old really fast.
Thanks for mentioning this, it'll probably save me a few hours in the future!
Basically it presents ISOs over the USB connection as if they were storage devices, allowing them to act as boot media for a connected PC.