As someone who used to be a lifeguard, it’s 99% boredom and 1% pure adrenaline. I only had to jump on once and it was for a toddler at swim class. They didn’t have floaties on and were sitting on the side of the pool. The instructor was in the water with another kid with no floats and the one on the side of the pool slipped in and sank. The instructor couldn’t just let go of the other child so I jumped in and pulled the kid out. There was no thinking just doing. I didn’t realize until afterward I’d cut up my foot relatively bad when I reacted but couldn’t even feel it getting the kid.