When you take battle damage from an opponent's attacking monster: You can Special Summon this card from your hand, then you can Special Summon 1 monster from your hand with ATK less than or equal to the damage you took.
Special Summons another monster along with itself.
Requires you to take damage from an opponent’s attack to Special Summon.
Can only Special Summon a monster with attack less than damage you took.
Parry Knights along with the monster he Special Summons will most often be summoned during your opponent’s battle phase, which puts the Special Summoned monster in danger of being destroyed unless it has a high defense or protection effect.
Attack is the only restriction when Special Summoning a monster with the effect of Parry Knights. You can abuse this by Special Summoning high-leveled low-attack monsters. Monsters like Worm Victory have 0 attack in your hand but due to their effect, can gain lots of attack while on the field.
Deskbots are an archetype of low-attack monsters but are able to boost each other on the field to become much stronger. Parry Knight is ideally able to summon Deskbot 009 to use as a beater or to use with Deskbot 001 to make Deskbot Jet.