The intent here today is to approach Rx. So if you can't do the prescribed weight, use a weight you can do for 5-6 reps.
Watch Spealer tear through Fran in this video from the archives. He was the first person to do butterfly pull-ups; people doubted that it could be done with a straight pull-up bar. Now all CrossFit Games competitors do them. This was when CrossFit Park City was just a couple of friend working out in a globo gym.
Here's a cool video, an ad for Adidas and CrossFit.