How do I pull up this task manager?

[trance2002]

Every once in awhile I will do some unintended combination of movments and/or key presses and I am presented with a 2X4 grid of running applications. I have no idea how I pull this up on purpose. It may be part of the OS or maybe Advanced Task Manager. I usually pull it up when I am playing Robo Defense in landscape and I do have a tendency to hit the capactive button unintentionally but have done it other times in portrait. This seems like it would be a great quick way to jump to other applications other than getting into the ATM list (more steps to get there) but since I've only done it by accident I don't know how to do this when I mean to. Anyone know how to get this pop up on demand?