Kim said eight versions of the prototype sink were made, and no matter how high the pressure of the water, there were no splash backs.
Day told People it was a tricky balance getting the slope just right without severe angles so the water stayed in one place.
"Only the best fabricator can accomplish this," she said.
"These finishes are not your everyday selections. Every part of this design and renovation is top shelf, including the client it was designed for."
As Kim zoomed in on the sink, she explained how the sloping design allowed the water emanating from the taps to pool slightly and then drain.
Kim's explanation comes after her Twitter followers were left scratching their heads over its design.
"Can someone PLEASE explain the sinks at kim & kanye's house?" one person asked last week.
The seeming lack of basins under the taps led another to speculate the vanity was made from "high end marble sponge", adding: "What, poor people don't have any at home?"
Another suggested no one knew what it meant "but it's provocative".
Kim's renovated "minimal monastery" mansion in the Hidden Hills enclave is worth about $60 million.
