If you leave the valve open, and then cap the flow, then the hot and cold cross over through the valve and mix in the walls.
Pretty soon you you hot water to the toilet.
If you were to try this in an apartment building, it would "cross over" the entire building.
I sell some shower head set-ups with multiple heads, and when you shut off or divert a head, it weeps a little water to prevent cross over.
What is wrong with using the faucet and turning off the water while soaping up?