[detroitcoder]

Actually you can but it is a bit clunky. Go to Browse > Dag Runs and select the 'Create' tab. This pulls up a form where you type in the Dag ID, enter the start time (set to now but keep in mind it is the local time of the web server), and a Run ID.

Totally agree there should be both a simple Start Run and Stop Run button.

[tedmiston]

What I've been doing when debugging is flipping the DAG switch to off, clearing the first task in the DAG so the whole thing re-runs, then back to on.