Pretty interesting, I read up a little more on it and watched a couple of YT videos. Every June a new tripod is built, so the one I saw last year was brand new. In March, they take it apart, drag it out on the ice and put it back together. It's made out of spruce logs (I thought it was metal...). A cable is attached to the top of the tripod and runs to the tower on the bank. It has a set of pulleys, so the cable is actually rather long, with a bucket at the end. The tripod does not have to fall into the ice for the contest to end, the cable just has to be pulled far enough, around 100 yards, for the bucket to be pulled all the way to the top of the tower, that sets off a siren and stops two separate clocks keeping time. The video below, the ice from upstream finally caught the cable and dragged it far enough to raise the bucket, well before the tripod ever finally went into the water. Pretty cool.