The half-volley, struck just after the ball has bounced, produces the hardest and flattest shots in football. It is also the finish that receives the least attention in training, because the timing window is unforgiving.
What the bounce contributes
A ball rising off the turf is moving upwards and towards the striker. Meeting it at that moment means the foot is working with the ball's own momentum rather than against it.
The energy already in the ball is added to the energy in the strike, which is why half-volleys leave the foot faster than shots struck from a still ball.
The strike also compresses the ball against the ground for a fraction longer, and that contact time is what turns swing speed into ball speed.
Why the ball stays low
Contact happens when the ball is only an inch or two off the surface, so the foot necessarily meets it above its centre.
Striking above centre imparts a downward or flat trajectory, which is the opposite of the problem strikers have with a dropping volley.
Goalkeepers dislike this shot for the same reason they dislike any low, fast strike: they must get their whole body down while the ball is arriving.
The timing window is measured in fractions
Too early and the foot arrives before the ball, catching turf. Too late and the ball has risen, turning the strike into a scuffed lift.
The margin is a fraction of a second, and unlike a volley there is no visual cue at the top of a flight to time against.
What players time against instead is the bounce itself, which requires reading the ball's descent several yards earlier.
Why it is neglected in training
Finishing drills use served balls, and servers naturally deliver flighted or rolled passes because those are repeatable.
Producing a consistent bounce at a consistent distance from the shooter is fiddly, so the half-volley is rarely isolated as a drill.
Players who strike it well have usually built the timing in unstructured play, where balls bounce awkwardly and nobody resets them.
Where it appears in matches
The half-volley shows up on cleared corners dropping to the edge of the area, on knock-downs from a target man, and on deflected passes.
Those are all situations where the ball reaches a shooter unplanned, which is precisely why the finish cannot be rehearsed into existence.
A player either has the timing available when the ball drops or takes a touch and loses the chance to the defenders arriving behind him.