The full syphon is also interesting - it consists of the main pipe, together with a larger pipe that fits over the top
The top of the clear section sits just above the water level in the weir which means that the water entering the syphon must come from the lower level of the weir - the documentation state that this results in greater mixing of the water in the weir and prevents 'dead' spots. I'm not sure if it makes a great deal of difference but there is a bit of logic to what they are saying.