Dropping your shield takes a certain amount of time. For most characters, 7 frames. So if you want to use an attack that isn't an aerial, Up Smash or Up B, you need to wait 7 frames while your shield drops, and THEN you can use that attack.
When you powershield in Melee and vBrawl, the hit lag and stun stays the same. However, you become able to do any attack during those 7 frames you should normally be lagging from dropping your shield. Your frame advantage hasn't improved - you still have to wait the same amount of time to become unstunned - but now you have more options. You can jab, tilt, or use side smash/down smash as early as you're allowed to grab or jump, without waiting for shield to drop.
However in Brawl+ there is a code that increases the amount of shield stun, because vBrawl has very, very little. That code didn't affect powershielding. So if in vBrawl you were stunned for 4 frames, and in Brawl+ you're stunned for 10 frames, powershielding in Brawl+ only gives you 4 frames even though it should normally give 10 frames of stun. Besides having more options, you're able to act sooner. The debate was whether or not that extra frame advantage was something that needed fixing.