Narrow path: no overtaking.
A square on the path that makes the path appear narrower. Monsters may not overtake one another on that square.
The main purpose of this is to make sure a Rockster is in front of any Ballys so it takes the fire from Gunshots and Laserguns so it is not wasted on the Ballys. It also slows the Ballys when that happens so they are vulnerable to any Spikes on the previous square or any Guinshots or Laserguns behind them.
It might be it is not effective 100% of the time and so has 'Effectiveness' as an upgrade %age.