Doing it standing up?

[Muskett]

The Target holds should have the rifle on target, with a natural body wobble. Allow for normal "prone" breathing cycles. All the right alignments in place, and first stage of trigger taken up. At some point, usually on breathing out, everything is in place to release the shot. This time max concentration to hold that extra steady. Practice well and "perfect" aim is given the extra milliseconds to release the shot. It is not a snap, but deliberate. Those practiced and heart rate sorted can extend the millisecond longer than novices, so making the shot deliberate.

On both Target and Sporting, stance should be correct, breathing cycles done, trigger control familiar with. More deliberate than a snap and yank. On the sight picture, then concentration can give steady on target, and not a release on a swing pass of the target.
Sporting techniques can either be hold or snap. Snap is on target, stop, and fire. Best used on a x1 or Single Point no mag scope, or just bead iron sights, so giving the fastest let off. Excellent for getting central hits, but not precision.

If just a hit, or requiring precision, really depends on target size. Most air rifle shots, be it on targets or game, require precision, so holding the aim is needed. Practice extends the milliseconds to hold on aim. Practice and concentrate enough then all becomes "zoned in" where it seems there is is plenty of time to release a perfect shot. Easier said than done. Don't forget to breathe before.......and after follow through..

Have fun...