Launchpad question about function

[EndlesTweakin]

Hi all

First post. Great to be here.

On the Launchpad, I'm trying to work out how the second function button works when in the Rack Edit mode?

From the Launchpad reference sheet, this function is meant to be "Play". I get a bunch of lit up pads and can have fun playing with them while the sequence is playing, but I can't work out the method to this madness.

Please help

thanks