The route stem is the initial vertical segment of a receiver’s route, consisting of the part of a route before the receiver makes a cut. This terminology originates from the “route tree”, a grouping of different routes (traditionally numbered 1-9) that are taught as the basics of receiver route running. The route tree consists of the branches, which are the breaks a receiver can make, and the stem, the initial section before a break is made. The stem of a route sets up the break that a receiver will make.
When stemming a route, the receiver will attack the defender’s hips and manipulate the defender into a poor position. The receiver’s goal is to get the defender off balance and put him in a poor position to defend the route he is running.
Against off-man coverage, Jones quickly eats up space between him and the defender. Because he runs the stem of his route so quickly, the defender must turn his hips to cover Jones vertically down the field. Jones also sells this well by breaking slightly to the inside when he gets to Darby. The defensive back reads this and commits to covering an inside-breaking route. Once the defender has committed in this direction, Jones stops on a dime and cuts to the outside to make the catch. Jones used the stem of his route to fool the defender and put him in a position where he could not make a play on the ball.
In the second example, Julian Edelman uses his route stem to put the defender off balance and to win leverage over the defensive back.
Edelman starts his route as an over, stemming it to the middle of the field. Once the cornerback is committed to defending this route, Edelman breaks it back to the outside. His stem put the defender in a poor position by making him commit to defending one route before running a different one.