Send in the Triarii?

So I noticed with interpenetration rules tonight...

"Blades recoiling into Blades or Spear"

Is this so that you can do the ol' Roman rank switch with your Blades and/or Triarii?

i.e. A line of spears facing off against the Romans (blades with a Sp/Triarii providing backup)

SpSpSp
BdBdBd
__Sp__

Left and Right combats are a push, but the central one ends up with the Blade losing...

SpSpSp
Bd__Bd
__Sp__
__Bd__

The Roman's next move is to march the Sp/Bd forward and now...

SpSpSp
BdSpSp
__Bd__

Or am I completely on crack? Is that what the various interpenetration rules are intended to represent? Is it supposed to be some kind of Medievil army that I have no idea about instead?

If I'm wrong, how does one go about using your Triarii? Gap filler? Or can you use interpenetration to your advantage? I guess I know how to do it with Ps, but even Bw can move through Bd...

Thanks in advance,
Chris