audacity gives me a 'double headed arrow'

When I am editing something in Audacity and I click on the timeline, sometimes I inadvertently get a 'double-headed arrow' instead of the normal down arrow. Once I get this it immediately starts 'playing' from the current position towards where the double-headed arrow is. That may be forwards or backwards but is definitely what I want to happen.
I assume that I have accidentally enabled some feature - but I don't know what that feature is. I would like to turn it off again. Any ideas?