I've seen players reflect lasers with just the normal shield. Is there a special button combination?
There is 2 types of Powershielding. When you enter full-shield, things happen that way:
Against Projectile :
Against Physical Hit :
Also if your Full Shield get hits by a projectile on the 5th frames, 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: