This card is very useful at the start. If none of the ground lanes are covered, this can normally do 6 damage to the zombie hero. However, this card is also useful for getting rid of pesky low-health zombies. In certain missions, all of the lanes are ground lanes and if you have this, use this as soon as possible as you will strike down 10 health of the zombie hero if not blocked.
Do not use this if the ground lanes are occupied by zombies with Armored 2 or they will go unaffected, wasting the superpower.
Against
If Grass Knuckles has this superpower at the start of a battle, then there isn't much you can do, forcing you to accept at least 6 damage (-2 damage per zombie in the ground lanes) dealt to your hero. If he has it later in the game, however, make sure you have a zombie in each ground lane that have high health, therefore they will take damage instead of your hero.
The description refers to the term "knuckle sandwich," which is a slang term for a punch in the mouth.
This may be a nod to Bonk Choy's Plant Food effect from Plants vs. Zombies 2. Using Plant Food on it will deliver many punches in an area around it, similar to what Grass Knuckles does here.