All monitors (sinks) have EDID (Extended Display Identification Data) registers in which the information regarding their supported resolutions is stored. The source reads this information over I2C to determine the format to transmit. When the monitor is in stand-by mode, the source still needs to read that information and hence powers the bus with 5V as the monitor is in standby. When the dongle comes in between the source and the sink, it becomes the responsibility of the dongle to power the bus with 5V and read the EDID data and give it back to the source.

