It gets its name from the confectionary product 'Curly Whirly' and is when a male ejaculates upon a females/males arm when recieving a hand job. This is related back to the 'Curly Whirly' as there are thin strands of excess chocolate on the product which emulate the cum on the females/males arm. The desired effect depends on how much you 'blast out of the cannon' and the proximity it lands over the arm of the recipiant.
"This babe was feeding my chicken last night and I left a Curly Whirly on her arm. It was so ace, man!