My go!! Lets assume you are in High St and turning right into Main Road and its a yellow box junction. If there is no backed up traffic in Main Rd, your exit (that is the exit from the box junction)is clear. Therefore you can proceed into the box and wait for the oncoming traffic to stop before proceeding into Main Rd. If there is a tail back in Main Rd and you would not be able to turn in to it, clearly your exit is not clear, so you cannot enter the yellow box. The reason for being allowed to wait in the box to turn right is this. Imagine you weren't allowed in, as soon as the lights went red and stopped the oncoming traffic, you would have zero time in which to perform the right turn, so you would never get to do it unless there was no traffic coming the other way. This would be frustrating to say the least!