It's not an intuitive interface, but it works -- use the slider to slide the price downward to zero. It's a variable-pricing model with varying payout to the author.

(I'm not criticizing the pricing model, I'm just saying the free-purchase method wasn't obvious to me, either, at first.)

I bet you'd get more ultimate conversions if you stated that full-price purchasers would be granted the full book upon completion (and correction/minor update versions as well).

that may be a leanpub one off error