So, the tennis balls: in your hands to make it harder to swim (kicking workout?). I'm also envisioning a Retriever dog-paddle tennis ball carry.
Funny, I was picturing your coach throwing tennis balls at you while you sprinted your brains out.
Thinking indicative of some of my experiences as a competitive swimmer (substitute tennis balls for kickboards) :)
I thought you threw them yourself. Inquiring minds want to know.
Throwing them at each other certainly would be more fun.
First set fr and 2nd set fly/br was with one under your chin. Makes you rotate further to breathe with fr and keeps your head down with fly/br (or in my case makes you inhale H2O).
3rd and 4th set we had one in each hand. It helps you focus on pulling your forearm through the water correctly by eliminating the use of your hand. There is some subtle cheating occurring here I suspect. It is amazing how adding in just your pinky and ring finger can help propulsion. Not that I would know for sure.