Hawaiian Squats were a new addition in 2014 to the Spartan Race stadium series of obstacles. However they aren’t actually a hawaiian squat, but more on that later. What Spartan Race is using as the Hawaiian Squat obstacle is you approach a row of stadium seats, and proceed to sit and stand up on every seat in the row. This is actually a chair squat and not a proper Hawaiian Squat. If you go to the second exercise on this page, they clearly show what a real hawaiian squat is and it is much more challenging then a chair squat. My guess is that since this obstacle debuted in Hawaii, and is a variation of a squat that the name Hawaiian Squat stuck.
I would say there is no need to practice specifically for this obstacle as chair squats shouldn’t be very hard if you are doing a Spartan Race. But some basic practice would be to do a few sets every day of air squats or chair squats for beginners. And for photos of the obstacle see here: