Almost every recurring finishing fault traces back to the foot that does not touch the ball. Where the standing foot lands sets the hips, the balance and the available angle before the shooting leg has done anything.
The hips follow the standing foot
Plant the standing foot pointing at the near post and the hips square to it, which limits how far across goal the shot can be steered.
Plant it pointing at the far corner and the hips open, and the whole far side of the goal becomes reachable without contorting.
Strikers who always finish to the same side of the goal are usually planting the same way every time rather than choosing the same target.
Distance from the ball sets the trajectory
A standing foot too far from the ball forces the shooting leg to reach, which drops the shoulder and lifts the strike.
Too close and the leg cannot swing freely, producing a jabbed contact with no follow-through and very little power.
The workable distance is roughly a hand's width to the side, close enough for the head to stay over the ball and far enough for the leg to travel.
Front-to-back placement decides height
Planting level with the ball or slightly ahead of it keeps the body over the strike and the ball down.
Planting behind the ball, which happens when a player arrives a stride short, tilts the torso back and the shot rises.
This is why a striker who misses high in a match and never in training is usually being rushed into arriving early or late, not losing technique.
It absorbs the landing as well
The standing leg takes the whole body weight during the strike and again as the player lands, often while a defender is making contact.
A poorly planted foot means the striker is falling before the ball is gone, which is the real explanation for shots that squirt sideways under pressure.
Strength work for finishers concentrates on single-leg stability for exactly this reason rather than on shooting power.
How to see it in a replay
Watching a missed chance again, the informative frame is the one just before contact, not the contact itself.
If the standing foot is behind the ball, the shot was going high before the leg swung. If it points across the body, the angle was already closed.
Diagnosing finishing from the shooting foot alone explains very little, because by then the outcome has largely been decided.