Ah yes, I remember watching this; mantids will only eat live prey so keeping them as pets means you see this daily.
It's actually fascinating to me how their mouths work and I would watch them eat just to figure out their mouthparts since it seems all the tiny parts move independently to eat their prey... I'm just glad that the lack of pain receptors in a lot of their prey items means that they don't suffer as much as if they ate something with more of them such as small birds, small mice, fish, ect...
Precarious333 on youtube has some amazing high definition footage of various species doing various things; hatching, eating, threat displays, ect.
They're designed for it, the mouthparts just take a small chunk at a time off the prey (while still alive) until they consume the entire thing. They're also cannibalistic, eating other mantises the same size or sometimes even bigger than them in the more aggressive species such as Creobroters. I had a few batches of babies and they have so many babies, that it is impossible to separate them all into a mantis-per-enclosure setting or you would have hundreds of kritter keepers or tanks piled up; unfortunately that means they get clumped into a handful per tank which leads to.. disappearances of the weaker ones, no matter how much you feed them; another mantis is simply a more attractive meal than a fly half their size.