It's nerve-wracking and just a little masochistic. Yarn chicken is when the design you're knitting calls for a certain amount of yarn, and you're just not sure you have enough.
Maybe the skeins of yarn don't quite have the same yardage as used in the design. Or maybe you used a little bit for another project but have most of the skein left. Or you decided to change up the gauge or sizing or something else.
Designs are usually written to call for a bit more yarn than needed, in order to account for some waste yarn when different people knit things up.
You decide to take a chance and knit up the design in that yarn. You cross your fingers and cast on. The game begins. Will you be able to finish it? Or will you run out of yarn and have to change the design some more to make it work?
Oh yarn chicken - you can be a heartbreaker.