I had a few NST's at the end of my high-risk pregnancy because I was contracting (but not dilating, ugggggghhhhh). It's just as Dee Darling 3 describes, you lay on your side and the put on an external monitor to see how baby is doing. If the baby is being super quiet and they can't detect movement, they might use a sound thing to get the baby to wake up--Maggie was napping at one of my NSTs and the sound thingy totally freaked her out and woke her right up!

It's a hassle to have to go in twice a week, that's for sure, but at least an NST isn't invasive or anything.