Roller Coaster Power Cut

A power cut in Arkansas left roller coaster riders dangling upside-down My first reaction is 'how'. My understanding is that roller-coasters are constructed such that once lifted up the climbing hill, the coaster proceeds under gravity. Given that, how could a power cut cause it to stop in the position described?

The big worry for me if there were a power cut whilst I was in a coaster would be the braking system, not gravity ceasing to operate normally!

So, how could this happen?