A Snappi is a plastic T-shaped grip used instead of pins to securely fasten folded flat nappies. Snappi are a very clever invention: just a plastic T shape with little claws at each of the three ends. Simply hook one claw into the flat nappy fabric roughly over one thigh (while holding the nappy together with the other hand), stretch over to the other thigh and catch into the loops of the fabric there, then pull the last claw down to catch at the fabric between the legs. The resulting grip should be Y shaped if it has enough tension.
Snappis are much faster to use than pins, and provide a nicely balanced secure fastening, which gives the nappies they are used with a high degree of containment. You have total adjustability both up-down as well as left-right, it is hard to beat the fit that Snappis offer.