Running specs in a separate terminal

I’m trying to set up vim-rspec in a similar way to most recent Weekly Iteration episode, so that I can see my code and test output at the same time.

I’m using terminal Vim inside iTerm. I’ve set let g:rspec_runner = "os_x_iterm" but it doesn’t seem to have had any effect.

The README says “This will run the specs in the last session of the current terminal” but I’m not quite sure what that means, should they run in one of my open iTerm tabs?