Shots that fly over the bar are usually blamed on nerves or on trying too hard. The physical cause is simpler and more correctable: the head came up before the foot went through the ball.
The head leads the trunk
Lifting the head pulls the chest and shoulders backwards. That is not a stylistic detail, it is how the spine and the muscles along it are arranged.
Once the chest is back, the hips follow, and the body's centre of mass sits behind the ball rather than over it.
The foot then makes contact on the underside of the ball on a rising path, which is exactly the strike that clears the crossbar.
Why players lift the head at the worst moment
The instinct is to check where the goalkeeper is standing. That check feels necessary, and in the final half-second it is the most expensive habit a finisher has.
Good finishers gather that information earlier, in the approach, and then commit to a target without looking again.
The scanning happens before the ball arrives, which is why elite strikers appear to be looking around constantly in the seconds before a chance and perfectly still at the moment of the shot.
Keeping the head over the ball
The coaching phrase is to get the head over the ball, and it describes a body position rather than a stare.
With the head forward, the chest closes over the contact point, the ankle can extend downward, and the strike passes through the middle of the ball.
The ball then leaves the foot on a flat or descending path, which is the shot goalkeepers find hardest because they have to get down rather than up.
The role of the standing foot
Head position and standing-foot placement are linked. A standing foot planted too far behind the ball forces the body to lean back to reach it.
Planting alongside the ball, or very slightly ahead, lets the head stay forward without the striker feeling as if he is falling.
Strikers who chronically shoot high are often simply arriving a step short and stretching, and fixing the run solves the shot.
Why volleys punish it hardest
On a volley there is no ground contact to reference and no time to correct. The body position at contact is whatever the approach produced.
A volleyed shot with the head back gets airborne immediately, which is why the ones that stay down look so much more impressive than they were difficult.
The best volleyers lean into the ball and let their head finish above the strike, accepting that they will land awkwardly afterwards.