Legato is a technique to play the music by connecting notes to one another, without interruption (the nots tied together). On the principle of legato playing, students must maintain the sound previously with relaxed and without releasing the fingers on piano keys. This was repeated on subsequent presses keys. When a key is pressed next, then the keys must be released earlier, but cultivated an uninterrupted sound.
The important thing is that although previously suspended because the finger must remain on the keys, the previous motion should not be hampered. So tried to your fingers remain relaxed and use the optimal movement, accompanied by wrist and arm as one unit.
Imagine if it was all played on a guitar string:)
Watch the video to get the real impression on how John did it..
John Petrucci - Legato Lesson Download to mp3