Strafing - MegaMek/megamek GitHub Wiki
Strafing is in Total Warfare and has complex set of requirements.
If you're at the correct altitude (1-3), one of the buttons for the ASF's firing will highlight ('Strafe'). By clicking this, you can then designate hexes to strafe (1-5 of them, in a straight line, must have flown over them).
Once you've done that, you need to select your energy weapons and fire them (only energy weapons with infinite ammo can fire strafe).