If you look at the way they are set up really close, it seems they would be able to drive out from under you truck (rotate) fairly easily. I don't see anything keeping them right side down but the weight of your truck.

 

its probably a mixture of the truck's weight and the tracks are probably on there really tight.

 

My method, if there is room, is to walk along the hole and poke around with a stick first if I'm unsure about what could be lurking. Also to check depth if I've never been through before.

 

lol yep you never know when your going through a puddle and the water is coming over your hood... that's happened to me a couple of times, makes your ***** go to the bottom of your feet

 

So far I've only misjudged one hole. Deep spot where couldn't reach with the stick. Luckily it only got the floor wet. And my feet..