String, properly used, ensures uniformity of cooking and creates an appealing appearance. If an entire loin or roast is being cooked, string can also make carving easier. Finally, though this is not fine enough for a first-class kitchen, the tapered part of any cut, such as in a tenderloin, can be folded over and tied to create a perfectly cooked extra portion. Hold string with thumb, index finger and middle finger of left hand as shown. The slipknot is what makes string so useful.
Loose end goes over standing end, over itself, then under standing end.
Whatever you do, don't try to teach yourself to tie a slipknot five minutes before the guests arrive or you'll throw your bolt of butchers' string through the window and never touch it again. This is something to practice on a weekend afternoon. It might sound odd, but once mastered, tying slipknots is hugely satisfying. Tied roasts and racks look beautiful when properly seasoned with plenty of salt and pepper and sprigs of thyme before being slipped into a hot oven. They become especially pleasurable to cook because you know they'll be perfectly done all the way through.