The piece at the bottom of the picture is the elbow right off the engine... at the manifold if you will. Notice the remains of the old exhaust gasket. Hot exhaust gas goes in there and up that straight riser to the short side of the mixing elbow. The small fitting on the mixing elbow is for water injection... what you don't see is the inside of the mixing elbow... in there the injected water is forced to the right by an internal dam. That dam runs across the inside at the top and to the right, with the opening on the right side of the mixing elbow. The water is forced to the right and to the downside of the elbow away from the engine, and the source of the hot exhaust gases. The gases and water meet just above the longer leg in the mixing elbow.

In the full installation, the dry elbow is bolted right onto the engine, and the bend points upward, with that riser going straight up. The mixing elbow is at the top, and thus the raw water is injected in a manner that it is on the low side of the mixing elbow (in spite of where the fitting really is).

I suppose a failure of the metal in the mixing elbow at the back of the "dam" where the water comes in (and is redirected) could allow a leak down the riser into the engine. But I suspect that metal would be thin in other areas too and warn you of this.

The way it failed on me was that right where the gases meet the cooler metal, at the top of the bend, rust, salt, and exhaust "coal" all formed much like condensation on a cool glass... this made a rock like substance very similar to fire brick or lava rocks in a gas barbeque.