The jumper for selecting whether the I2C grove port is constantly powered or switched powered was not intended to allow the user to turn off I2C devices on that Grove port. I designed the board that way because I wanted to give users at 2 separate Grove ports that could be powered either continuously or switched, because the other 4 Grove ports only give you the option of 3v-switched or 5v-switched. To avoid confusion, I probably should have only allowed people to choose switched power on the D4-5 Grove port and made the I2C Grove port be always on, but on earlier versions of the board, I was using a sketch that turned on and off the power to the I2C Grove jack because of a certain peripheral I was using on that jack. So the jumper is basically a relic from an earlier iteration of the Mayfly that has limited functionality to most people, but it’s there if you need it for a specific reason. Almost all users will leave it in the default 3.3v constant position.