Explain this code to me plz

I posted a few threads back and I think I might have what Im looking for.
Im not totaly sure how I got it to work but it works.
Could some one explain to me how this code works.
on (rollOver) {
Its placed on the tab that activates the menue. On my stage I have 3 frames

  1. the activate tab
  2. for the menue going down
  3. for the menue going up
    I thought that the code would only tell the tab to go to frame 2 and stop. But as you can see it also plays frame 3 when you rollover it again.

Does it work like this
If the time line is on frame 1 then go to frame 2.
But if time line is on frame 2 then frame 2 is 1 and thus go to frame 3 represented as frame 2.
thanks cloak.