On the left is the longer leg looking at you, in the darkest region there you see the water "dam." On the right is where the hot gases enter. Just below the large opening on the right, is the fitting for the injection water, which is kept behind that cast piece around the elbow until it comes out on the left, thus it forms a "water jacket."

I could see a big failure area right at the cast line... if that were to rust through.

The old one was blocked at the inside bottom of the elbow, right where the little sliver of light is showing in this pic.

I couldn't figure it out at first either as I thought the water just went right into the elbow... the built up one I have has fittings that don't allow me to see all these features, so I couldn't tell how the water jacket (dam) worked. For all I know, there may be something in there. I know water flows through it though. Because I cannot inspect the whole thing is one reason I bought a new one... Also there is quite a bit of residual build up still left in the old one.