How can I use the shield to reflect projectiles?
There are 2 types of Powershielding. When you enter full-shield, the following happens:
- The first 2 frames are ineffective
- The next 2 frames are used to Powershield projectiles and physical hits
- The next 2 frames only Powershield physical hits
Against Projectiles:
- Time Window is 2 frames
- Knockback is cancelled
- Hitlag is cancelled
- Projectile ownership becomes yours
- Projectile damage is halved
- Projectile is sent back in the opposite trajectory
Against Physical Hits:
- Time Window is 4 frames
- Knockback is increased
- Shield is cancelled (means you can do anything you want after Powershielding)
Also if your Full Shield get hits by a projectile on the 5th frame, it won't get Powershielded, but as long as you remain in shield mode, the next physical hit you receive will be Powershielded, even if you switch to Light-Shield.
This is called a perfect shield and happens when you throw up your shield at the exact right moment. It's incredibly hard to execute and requires a lot of practice. An excerpt from the linked wiki page:
Perfect shielding is done by quickly and fully depressing a shield button two frames before an attack connects. If done correctly, there will be a significant flash on the shield and a distinctive "chlink" sound. Because it is technically a shield, it is ineffective against grabs.