While the equipped monster is attacking, it cannot be destroyed by battle. At the end of the Damage Step, if the equipped monster attacked: The attack target loses ATK equal to the ATK of the equipped monster, until the End Phase.
Rocket Warrior is a good card to equip with this card, while you doesn’t receive any Battle Damage you can reduce your opponent’s Monster ATK even more. Amazoness Swords Woman is another good card to use. while battling High ATK monster, instead of reducing your opponent’s Monster ATK, you can reduce your own Amazoness Swords Woman and inflict the battle damage to your opponent.
Not sure why people think this will be particularly good, it won't. It's one of those cards that, in theory, seems good. But in practice is rarely useful. Covering Fire's a similar card, for similar reasons. Being in a position where you have 2 attacking monsters while the opponent has something big enough to need to weaken it isn't very common. And Dark Core's probably a better out in those cases anyway.