Pedra Que Engole means "swallowing rock," an accurate name for a small Brazilian cave accessible only via waterfall.
There are some great first-hand reports on TripAdvisor, like this one from J Pedro O:
This starts with a hike (that took me a bit to find where it started). The hike itself is really beautiful. You go through the rainforest for around 20/30 minutes until you reach a small waterfall. This is really nice to cool down in before going up to the rock. You walk up after 1 minute and you see several rocks. The rock that you go under is between two big rocks, where the water is falling down between. You can see a hole there slightly. Put your arms straight up and legs straight down and let the current take you under the rock! Its really easy, but I did scratch my back slightly. Under the rock it really is beautiful as well. I loved it so much I went 3 times! A MUST in Paraty