Simple question. You'll all know this ...

Can't work it out ...
On the timeline track header there's an icon next to the padlock icon that I can't identify.
It's a rectangle with two outwards facing arrows inside of it. It doesn't seem to do anything and doesn't reveal its function when you hover over it. Its not mentioned, as far as I can see, in the manual. Please put me out of my misery.
On the timeline track header there's an icon next to the padlock icon that I can't identify.
It's a rectangle with two outwards facing arrows inside of it. It doesn't seem to do anything and doesn't reveal its function when you hover over it. Its not mentioned, as far as I can see, in the manual. Please put me out of my misery.