You have to act instantaneously.

His reflexes acted instantaneously, a tautening of every nerve even before his flesh had started to creep.

One glance was sufficient for her to comprehend an entire situation, and to act instantaneously in accordance with what was expected of her.

His theory assumed that gravitation acts instantaneously, regardless of distance.

Such forces, acting instantaneously at one place due to circumstances somewhere else, are called non-local.

It is now thought that some unseen connection binds them, defying known physical laws, acting instantaneously without reference to the speed of light or any other limit.

Your body can effectively lower the force by compressing a bit too, so actually no acceleration really acts instantaneously.

A drug should act instantaneously and brilliantly.

And he acted instantaneously.

It acts pretty well instantaneously.