What does a Chroma key ring do?

What does a Chroma key ring do?
http://www.bhphotovideo.com/c/product/489495-REG/Datavideo_LD_1_TD_2_LED_Dual_Color_LED.html
Just a little bit more info.
I know what the Green screen sheet does....But what does the ring do that goes around the lins of the camera?

Its like the news when they replace the green screen withe the weather,,Puts a image in an image on the green background..
Thus Chroma key..

http://beta.searchme.com/Initial.html?q=Chroma+key#/0/&pi=0/&q=Chroma%20key/&ci=all/&session=970BE3BB76DAA3CB1818DB71381E3FEDE65CE15F/
http://beta.searchme.com/Initial.html?q=Chroma+key+ring#/0/&pi=0/&q=Chroma%20key%20ring/&ci=all/&session=F2D52AFB4A942E7DCB297463472998EB3A5C02D2/

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