In local dev:
`rake test`
`rake coverage`

Rake coverage will generate up to five coverage files depending on the areas you touch. Grab the paths of the coverage files that are generated (all will be `reports/...`) and run

`diff-cover report/... report/...` 

listing all of the reports. That will run diff-coverage on a union of your reports.

One of these Fridays I hope to have enough spare time to add that union into the rake coverage task itself.