Break a defence with an underlap and cut-back. Fix the defender with the ball, release the inside run into the half-space, drive to the edge, and cut it back for a runner to finish.
A winger and a full-back combine, but the full-back underlaps inside into the half-space instead of overlapping outside. The winger fixes the defender and releases the inside run, the full-back drives to the edge of the box and cuts it back to a striker at the spot. Rotate roles, and mix overlaps and underlaps so the defence cannot read it.
Inside or outside is decided by the defender, not by the full-back's preference, and teaching both runs without teaching the choice produces players who overlap into a defender who was inviting exactly that.
The cue is which way the winger is being shown. A full-back defending an inverted winger will usually stand him up and force him wide, keeping his own body between the winger and goal. That posture leaves the half-space, the channel between him and the centre-back, unguarded, and the underlap runs straight into it. Overlapping in that situation runs into the area the defender is already protecting.
The underlap is also harder to see. A defender tracking the ball and the winger has the outside space in his peripheral vision, because it is on the same side he is facing. A runner going inside disappears behind him, which is why underlaps so often arrive completely unmarked and why the receiving pass has to be timed rather than telegraphed.
What it produces is a different chance from an overlap. Arriving in the half-space, the runner is central enough to cut back from a cleaner angle, and the pass across the box travels towards attackers rather than across their path.
The risk is worth naming. An underlap takes the full-back into a congested area, so a misplaced pass is turned over centrally, which is a far more dangerous place to lose it than the touchline. Against a compact block the overlap is often the safer choice.
Save it to your private drills, then change the animation, adjust positions, swap equipment, or drop it into a session block. Or start from a blank pitch and build your own.