Trinket is based on V-USB. It is a bit-bang implementation of USB. The generation of signals is done through assembly code, outputting 1s and 0s to the USB signal pins with precise timing.

V-USB is designed for AVR microcontrollers without native USB capabilities. USB signals have tight timing specifications that must be met, which is why the timing critical portions of V-USB are written in assembly code. Assembly instructions have predictable execution times and thus it is easier to calculate timing.

This is what makes V-USB so cool! Most of the microcontrollers on the market with native USB are big, but V-USB can turn a 8 pin ATtiny85 into an USB device!

To learn more about V-USB, check out the example projects on the V-USB website: http://www.obdev.at/products/vusb/projects.html

Trinket can do almost any of those projects, as long as there are enough pins and enough flash memory to use.

Beware however, like I've mentioned in the overview, V-USB is only capable of creating low speed USB 1.1 devices. If you need to create USB 2.0 devices, you need a microcontroller with native USB such as the ATmega32u4 (used in the Arduino Micro & Leonardo), not "bit-banged" USB.