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.
|Pack||Warriors Unite [SR]|
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.
You can Special Summon low-leveled Tuners to use alongside your Parry Knight to Synchro Summon. Here are some good Tuner options that can be summoned with minimal damage.