Phil Monahan demonstrates how to use a perfection loop for attaching a fly to your line. The perfection loop is better known for use in creating a loop in the butt section of leader but it can also be used to create a loop knot for nymphs and streamers.
A loop knot allows the fly to move more freely in the water, offering a more lifelike action. The nonslip mono loop is a good option, but some folks find it hard to master or two make with cold hands. The perfection loop is certainly easier.