* Gravity's Role: When your hands are lower than your elbows, gravity helps the dirty, soapy water flow *down* and away from your forearms and towards the drain. This prevents the dirty water from running back up your arms and recontaminating your newly washed hands.
* Contamination Prevention: The most important part of washing your hands is removing bacteria and viruses. If the dirty water flows back over your clean hands, you're essentially undoing the work you've just done.
In summary, keeping your hands lower than your elbows throughout the handwashing process ensures that the dirty water, along with the pathogens it carries, is effectively removed from your hands and doesn't run back up to recontaminate them.