This is an important "move" to learn. A shockwave will clear the screen of all enemies (excluding bosses), so it is invaluable in the latter part of the game. To perform a shockwave, move towards the middle of the screen in a straight line preferably, then use your time travel power to rewind time. Upon rewinding, you should shoot your time shadow. After several hits, it will explode, sending a screen clearing shockwave. This can be done from the very beginning of the game.
If you're having trouble shooting your time shadow before it disappears, move towards the middle of the screen, then backwards before rewinding.