Final answer:
The phrase 'advance the drain 1 inch' refers to moving the drain opening down by 1 inch in sink drains to increase the flow rate of water.
Step-by-step explanation:
The phrase 'advance the drain 1 inch' refers to the action of moving the drain opening down by 1 inch. This is commonly done in sink drains to increase the flow rate of water by creating a steeper gradient. By lowering the drain opening, the water is able to flow more quickly and efficiently through the pipes.
For example, if the drain opening is initially at the top of the pipe, moving it 1 inch down would create a greater difference in height between the source of the water (e.g. sink faucet) and the drain opening, allowing the water to flow faster.