Until now I had assumed that a "peep" was that squishy sugar-covered marshmallow treat that we enjoyed as kids and a "yo-yo" was a toy on a string. As it turns out, peep and yo-yo are also term used to described types of diving patterns. A square dive is one in which there are no excursions to the surface, known as a "peep", except at the end of the dive, of course. This is in contrast to a yo-yo dive in which divers repeatedly "peep" for a moment at the surface during a dive. The problem is that yo-yo dives are thought to increase the risk of developing neurologic decompression illness, which happens when bubbles in the circulation move from the venous side to the arterial side of the body by way of the lungs during recompression. These arterial bubbles can damage the nervous system and lead to paralysis. For this reason, yo-yo diving is considered a dangerous practice.
Interestingly, studies of small mammals, such as rats, have found that yo-yo diving helps prevent the development of decompression illness. But a recent study published in the Journal of Applied Physiology found that larger mammals, such as pigs, have increased risk for developing decompression illness when they underwent a square or yo-yo dive with 4 peeps. In contrast, a yo-yo dive with only 2 peeps was found to somewhat decrease the risk for decompression illness although the findings did provide evidence to justify the concern about this type of diving practice.
Ofir D, Yanir Y, Mullokandov M, Aviner B, Arieli Y. Evidence for the infiltration of gas bubbles into the arterial circulation and neuronal injury following "yo-yo" dives in pigs. Journal of Applied Physiology. In press. doi: 10.1152/japplphysiol.00392.2016