what and when is the catch?

Thanks everybody for your suggestions so far. Given that this sport is really my last opportunity to keep racing (due to other injuries), I want to make sure I don't blow it. Lately my shoulder have been feeling "funny"...I'm not quiet sure if they're sore, or have onset of injury. And, I'm suspicious that the reason is pulling too hard too early, and extending the arm too far. Many of you have mentioned pulling at the catch. In rowing, the catch is when the oar hits the water. What is the catch in swimming? And, does anyone know the guideline for increasing mileage? In running it's 10% per week. Is there a similar rule in swimming? Thank you all so very much for everything. Jerrycat ;)