It was a set up for slapstick, of course. Monica rushes to answer the door, taking her knitting. Hilarity ensues as Chandler is forced to follow with the skein and Rachel with the yarn ball since they are all connected and they get wrapped up in the yarn.

Monica (Courtney Cox) knitting with straight needles. She's holding her needles correctly and in the actual show you can see her making stitches. But what the heck is going on with Rachel and Chandler? Chandler on the left holds a skein over his two hands. a yarn strand goes over the table to Monica's needles. But in the background Rachel sits with a ball of yarn that she is also winding from Chandler's skein.