Before stepping into the shower, most bathers adjust the water temperature. If it's a tub and shower combination, the water will typically pour out of the tub faucet. An adjustment to the diverter valve will divert the running water from the tub faucet to the shower head. In some situations, the lever to control the diverter valve is part of the tub faucet. Lifting this type of diverter valve lever will send incoming water to the shower head, and lowering it will divert the water through the tub faucet. Without the diverter valve, the tub would need separate water control handles for each faucet.
The lever portion of the valve, which is visible in the tub or shower, might be a separate fixture or incorporated into the faucet or water control handles. When water is turned on, it will naturally come out of the faucet of least resistance. Depending on the position of the fixture's lever, the diverter valve will either block the lower faucet or allow the water to flow through it freely. When the valve blocks the lower faucet, it will force the water up the pipe to the shower head, against gravity.
Diverter valves come in a variety of styles, but the fixture's inner parts, which are installed in the wall, might resemble a piece of pipe with a shorter pipe fitting jutting out perpendicularly from its centre. Or, it might be a "Y" shaped pipe fitting that is visible. Aside from diverting water from the tub faucet to the shower faucet, the valve can also be used to move water between two different shower heads.
Over time, the diverter valve can corrode and will need to be replaced. The life span of the diverter valve will depend on how hard or soft the water is that travels through the pipes. One way to tell if the diverter valve needs to be replaced is if the water pressure through the shower is low. Another indication is a diverter valve that can't keep the water consistently moving through the shower head but sputters back and forth, pouring out of the tub faucet and then drizzling out of the shower head.