Why is my charge blade's shield glowing?

I started playing Monster Hunter 4 Ultimate recently. Being a primarily sword & shield user in previous iterations, I wanted to give the Charge Blade a try, as it seems to get most of the benefits of the S&S and add in a better source of damage with axe mode.
I understand the basic concepts of using the Charge Blade; charging phials in S&S and expending them in axe mode. However, on one hunt (probably due to some accidental button press), my shield started glowing red. Why/how did this happen? Are there any differences to fighting with and without this effect, damage-wise or in combo progressions?
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How do you charge the charge blade shield?
How to charge your sword - Condensed Element Slash: You can also transfer charge into your sword. You do this by pressing the Right Trigger + Circle or B, and then holding Triangle or Y during that animation. When the sword locks into the shield after a second, release the button and you'll unleash an overhead slash.How do I charge my shield mh4u?
Press R + A while in Sword and Shield mode to charge it.What's ACTUALLY Wrong With Charge Blade Right Now?
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