This technique can be based on a aerial Curve Mend or or an on-the-water mend, or both. I’ll start with the Curve Mend version. This explanation is set-up for a right-handed caster with the river flowing left to right.
Face down-and-across and then cast down-and-across. While the fly is still traveling to the target, make a strong Curve Mend down-current of your position with the apex of the curve toward the middle of the stream. In other words, the belly of the mend will be pointing away from you in a down-current position. You will likely find it best to shoot line as you make the curve in order to prevent moving the fly away from its intended target.
If done right, the section of line between the belly of the curve and the fly will fall straight across the current, thus creating the horizontal leg of the L. The section of line between the belly of the curve and the rod tip will fall down-current, creating the vertical leg of the L.
As the line comes tight, the fly will swim broadside (across-current), before turning fully at the apex of the belly. At any point, the rod tip can be dropped or additional slack introduced to make the fly stop swimming and begin drifting. This type of directed swimming action will often out-fish a fly that is simply cast and stripped back in whatever direction the currents dictate.
The same type of thing can be accomplished using an on-the-water mend by again casting down-and-across, slipping line to introduce extra slack, and then directing that slack into a tight L on the currents. For many anglers, this is easier to handle than trying to create a big Curve Mend. In my own fishing, I may use a combination of both, following an aerial Curve Mend with an additional on-the-water mend to generate a sharp, long-lived L.