Breathing to one side for one length, then breathing to the other side for the return length. This is something that Phelps/Bowman have discussed quite a bit. Oddly, before I even read about Phelps doing it, it was something that I incorporated into my own training several years ago. I just can’t keep a comfortable, consistent breathing rhythm breathing every 3, so I breathe every stroke. But I understood everyone saying that the merits of breathing every 3 are to prevent muscle imbalances. So to prevent that, I did the alternating length breathing thing. The added benefit of this is that when it comes time for doing a tri and swimming open water, I have the ability to switch my breathing side to adjust for waves/chop, sun direction, or someone swimming next to me, or sighting alongside land.
But doesn’t Phelps usually just breath on one side in races??? Looks to me like he only breathes on his right in this 2007 WR 200.