The Plastic Whip trick may seem difficult but in reality it is actually quite easy. This is a great intro to the "slack" style of 1A tricks.
You'll want to use a fairly unresponsive yo-yo for this trick.
First you'll want to throw a hard sleeper. After, you need to hold your hand in a "U" shape with your palm facing to your left (opposites apply for lefties) with the string running down from your string-finger and over your thumb. With a forwards-motion jerk, you want to create a loop or slack of string that will swing down forwards into the gap of the yo-yo. The yo-yo should end up hanging from the string looped around your thumb and string-finger.