Say I have a normal USB "Y" cable - female and male with data and power connections, and an extra male port with just power connections - normally used to draw extra power from an additional port on the host.
Would it be safe to, instead, use the cable as a normal USB extension cable, and use the power-only male plug to power a small external device, like an Arduino?
edit: It's a solution I suggested for someone who wants a simple, small device with a multicolor LED, attached to their laptop (very low power - it can certainly run off a USB port power, that's not the question). Using just a USB cord would mean losing one of the two USB ports on the laptop. Using a commercial USB hub would be bulky and impractical. My suggestion solves both of these issues, If it actually works safely. I don't necessarily think this is a good idea - I just want to know if it would work.
Maybe I should rephrase the question: It looks to me like the power connectors on the "power plug" of a USB-Y cable are simply connected to the power connectors on the "host plug" in parallel. This is designed to supply additional power, but as long as there are no diodes or other control circuitry, I believe it could draw additional power instead. My question: is this accurate? Is my understanding of the USB "Y" cable circuit correct?