Can someone explain how contractions look on the monitor?

During my NST, I was watching the contraction strip and there was bunch of pointed "mountain" like peaks and I was sure those were contractions! There was never a consistent flat line. Although I didn't feel much, maybe they were just BH or something? The nurse said I wasn't having any contractions regardless of the wonky looking strip. If you're having real contractions, how would they appear on the strip?

Edit: I am well aware of which strip is baby's heartbeat and which is the contractions strip.