Guarding
Guarding (aka blocking) is a core mechanic of Jcraft PvP that serves as the users most valuable source of defense in a Stand users arsenal.
Usage
A stand that's Guarding will give the player Resistance IV which is effectively 80% damage negation. A player that's blocking will also be provided with a heavy amount of Slowness as well, heavily reducing your mobility.
Guarding, because it gives resistance, reduces all incoming damage by 80%. This includes; Lava, Fall damage, and burning. However, void damage and starvation damage will ignore the resistance. As well as The World: Over Heaven's singularity attack.
A Stand that's Guarding will reflect any vanilla projectile that is shot at it. This is done in two different ways such as having an arrow deflected in the direction the player is looking, or a blaze fireball sliding down your stand until it reaches the floor.
Weaknesses
Guarding has a meter that must not reach 0 or else your Stand will die and need to be resummoned. This meter starts at 50% upon a Stand being summoned and quickly climbs to 100%. Damage dealt to the guard meter is proportional to the damage the user takes.
A player can't unguard during blockstun either. While this may seem like a benefit, there are times when you'd want to shift your hitbox or, as in Killer Queen: Bites the Dust's case, will disable your ultimate if you have it placed.
Backstabbing is another mechanic that creates a risk for blocking as if an enemy attacks the user's back while Guarding, they will go through the Guard and put the opponent in the appropriate amount of stun.
Irregularities
Some Stands lack the ability to block all together:
The Fool blocks all guardable damage during its block as it receives Resistance V. However, it's also slowed down to the point where the user is immobile while it's active.
Purple Haze, while capable of blocking, is uncontrollable and favors doing moves over blocking in a majority of situations.
The Hand and Cream, because they have moves that deal void damage, can ignore the damage resistance that block gives during some of their moves.
Cream can still block while the user is inside of cream, although it's less visually obvious to the user.