Sometimes you don’t really want to hurt someone, but instead need to hold them tight and stop them from moving. Make a Hard (TN 30) Physical Strength + Melee (Unarmed) roll. If successful, the defender is unable to move and may only struggle against their attacker. They may no longer make their own attacks or perform actions that require movement.
On subsequent rounds, the attacker may make a Hard (TN 30) Physical Strength + Melee (Unarmed) roll to maintain the grapple, opposed by the victim’s Physical Strength + Melee (Unarmed). For each round after the first the attacker receives a bonus +10 to the TN. For example, our hero has grappled a thief for 2 rounds already, so on the 3rd round he will receive +20 to the TN making it a Hard (TN 30) roll.
Every round that the attacker is successful at grappling, they may choose to simply maintain the hold, or block their opponent’s airways and asphyxiate them. Follow the rules for asphyxiation if they choose to do this.