That's how I understand it. It's not optional, but something that must be done once a month to keep your body guessing and metabolism fast.

You can choose to do the slingshot every 4th week, or you can choose to wait to do it until you feel your body going on a plateau. For my first 6-8 weeks or so I did not do a slingshot, because I was still trying to figure out the diet and remembering what things I could have when. Since then I've done the slingshot roughly every 4-5 weeks.

For a cruise, my recommendation is that you just use a "normal week." That way you can eat a little bit of everything while you are on the ship. Keep your portion sizes in check (use the hand sizes as a guide) and walk as much as you can on the boat.. assuming you don't hit the onboard gym. You'll do great!

Remember that a slingshot week is all high carb... so that means no "healthy fat options" including things like cheese, nuts, avacado, etc. if you want to follow the plan to the limit.