The scrub bar sits at the foot of the map interface screen in the Insight module. It looks like this:
By default the scrub bar is set to current time (i.e. the bar is coloured all the way to the right hand side). You can rewind up to 6 hours to replay the heatmap by "scrubbing" the bar to the left. The date/time in the black box above the scrub bar indicates how far back you are rewinding.
To return to real time, just hit the arrow button to the right of the date/time.
Note that if you want to go back more than 6 hours you'll need to be subscribed to our Analytics module (where you can select any time segment over for replay of the heatmap).