wipe off, press down with knuckle, to immerse
搵 centers on applying pressure or covering with a hand action: wiping off, pressing down with a knuckle, or immersing something into a liquid.
搵 combines 扌 (hand) with a phonetic component that includes 囚 over 皿. The hand radical suggests an action done by hand, while the right side likely contributed the sound. The exact historical development of the right-side component is uncertain.
The left hand 扌 works on the right side, which looks like a person 囚 trapped over a dish 皿. Imagine using your hand to wipe a dish clean, press a knuckle into it, or immerse it in water.
For オン, picture turning on a faucet to immerse a dish: on -> オン, and your hand wipes it clean under the water.