The word attack is way too harsh and forceful, literally and figuratively, to describe what your body does to get a piano key to go down…
But “attack” is so widely used in piano circles that using another word might cause confusion, so it will have to do for now.
Two Kinds of Attacks
A prepared attack means that your fingertip is already touching the key surface when you decide to give the key downward speed.
An unprepared attack means that your fingertip is above the key surface when you decide to give the key downward speed.
Prepared & Unprepared Attacks Compared
Each attack has a mix of advantages and disadvantages…
Because it provides tactile feedback at your fingertips, a prepared attack breeds enormous confidence that your fingers are on the right notes. The danger, though, is that you will start from a static position without any momentum and use unnecessary muscle tension to get the key down.
Because it starts above the key, an unprepared attack gives your arms, hands, and fingers some time and space to smoothly accelerate into the keys. While this cultivates a feeling of freedom and physical flow, the danger is that your fingertips will land in the wrong spot.
Which Attack Should You Use?
It depends on the nature of the musical patterns and your musical intentions. (Don’t forget: Technique & Interpretation are Inseparable!) In other words, it depends on the musical context and your personal preferences and playing style.
Whichever attack you use, be aware of the dangers and try to minimize them. The way to minimize the risk of missing notes when using an unprepared attack is to using your senses of vision and proprioception. And the way to eliminate tension when using a prepared attack is to have some momentum going in your arms and hands before the key goes down.