Take your DMM, or a bulb and a battery for that matter, and test continuity between the outside poles and the common middle poles as you flip the toggle.
When the toggle is in an up/down position ( moving in the vertical axis) and the toggle is down the middle row and top should have a connection, when the toggle is up the middle and bottom row should have a connection. Test with DMM to check you don't have a faulty switch.
So when the toggle is down poles 1 and 2 are connected, poles 4 and 5 are connected and poles 7 and 8 are connected.
Generally speaking though, the fastest way to understand this is to use a DMM and sit and test it, you'll understand it in five minutes.