I have two kitchen sinks, a P-Trap is indeed located under the floor. The problem I'm experiencing is that with heavy water flow (when open hot or cold water tap at full rate), the leak starts at "red rectangle" position. The strainer pipe fits exactly in the black plastic pipe, so there is no issue with size
I suspect there might be a blockage down the pipe, and as water goes back it leaks, but why would it not flow back in the sink then? When strain pipe enters the PVC pipe, is it supposed to have some plumber's putty/etc. applied?
I would be glad to have some advice on this. Thanks.
UPDATE As was suggested by Michael Karas, the solution is to install a compression washer on the tail pipe fitting in the black pipe.