Suppose you’re dialing across the 20-meter band one fine sunny afternoon and you hear a station calling from Mauritius, an island off the east coast of Madagascar. What a DX catch that would be! But after hearing the snippet of the station operator’s call followed by “QRZ,” your receiver gets blasted with a pile of strong, local transmissions, all in response to the Mauritius call. And some of the US stations go on and on for many seconds, striving annoyingly to make their call signs heard to Mr. Mauritius, the highly desired, rare DX contact. As a result of the cacophony of strong-signal responses in the US, no one was able to hear the relatively weak signal of Mr. Mauritius responding with one particular station ID that he desired for a contact. Everybody loses because of this operational inefficiency – even though contacts will be made within the chaos, a greater number of contacts could be achieved in the same time with smarter ops.
Read more at http://www.hamradioschool.com/g4a03-split-mode-ops/