When this skill triggers on a sucessful hit, your mech will deal the same hit twice: first to the opponent in front of him, and then to another enemy mech located in the front line, if there were another selectable target.
Example:
Your mech has Fork 30%. In a successful hit for 50 damage, your 30 % chance to activate the fork triggers, dealing not only 50 damage to the Mech in front, but also another 50 to another enemy mech placed in the same front line. (This effect won't hit twice the same enemy if there were a single mech in the enemy front line).
Tips:
If your mech had in its build some Fork skill, won't hurt to enhance it with equipment providing a higher Fork rate.
Focus on a few skills you want to enhance your mech and use as many pieces of equipment with such skills as possible. Also consider that a weapon with a rated damage lesser than another, may be better if included a high percentage of Fork. Especially if your mech already have a high possibility of forking the damage.
Notes:
Be aware that the mech can get this skill included in its build, as equipment or in weapons. If the skill was included as a Mech build skill or into the equipment, every weapon will have a chance to produce forked shots. But if the mech has not the skill nor the equipment, only when using those weapons with the skill may be a chance to get this effect.