It is the return throttle cable. Found this 08 forum but might help.

http://www.4strokes.com/forums/topic.asp?TOPIC_ID=29245


I have rode every year change of big bore xr since 1978 and always switched the throttle assembly to a single pull whirlpull and never experience any wide open or minor sticking. On my 2001 XR650R, I have left the stock configuration with both cables, works fine, just keep maintenanced. To me most binding comes from bad placement of throttle tube on handle bars; end of grips or throttle tube can bind on bark busters (hand gaurds), dirty throttle tube, handlebar, etc.

 

if theres 2 there there will be a push and a pull, the push isn't as necessary, just there to assist the throttle back to idle position, but then again as mentioned above bad routing or bad maintenance can lead to stuck throttles and such.