Process based parallelisation is simpler than thread based due to well, the GIL on MRI rubies and lack of 100% thread safety within the other gems. (I'm fairly certain for example that there are threaded bugs lurking within the mocks code).

Parallel testing in this implementation utilizes forking processes over threads. The reason we (tenderlove and eileencodes) chose forking processes over threads is forking will be faster with single databases, which most applications will use locally. Using threads is beneficial when tests are IO bound but the majority of tests are not IO bound.