Pulldowns seem to be the back exercise that novice trainees have the most difficulty properly executing. Probably the one being referenced here. It's very easy to use a lot of bicep.
to the OP or anyone else, here's a good trick to learn how it feels to really activate the lats using the pulldown. Add 50% to your pulldown poundage. Anchor yourself, reach up and grab the bar with a shoulder width grip, and then pull your shoulders down as far as you can without bending your elbows at all. The bar won't move more than about 6 inches, but the lats will contract throughout their full range of motion. Do an entire set that way. Bet you will feel it the next day, and it won't be in the arms.