My understanding is that ECS (and jive) developed out of Lindy back in the 1940s/1950s. ECS was an AM effort to "simplify" Lindy for audiences that weren't quite down with the kicks, throws, tricks, or basically "high energy" that was (and still is) popular with Lindy. While they are very similar, the shoes a follow wears help to put things into perspective: dancing ECS, I'm in 3-inch open-toed latin shoes. Dancing Lindy, women are in dance sneakers or something similar, with a much lower and sturdier heel. If I tried dancing Lindy as I've seen in on YouTube recently, in my heels, I'd break an ankle. Personally, I find ECS pretty tame compared to Lindy, or even jive. I struggle with ECS largely because it feels slowed down and blah compared to what swing "should" feel like to me.