What does the X button do Frankie?

Today, Bungie's Frank O' Conner hit up the NeoGaf forums to answer some questions people had and one of the question's people are asking is, "what does the X button do".  Frankie says that the X button will be used for the shield grenade and the trip mine that was shown in the VIDoc.  Below, Frankie talks about the graphics everyone saw in Bungie's latest VIDoc. 

"All of the water you saw, for example, is placeholder. No decorators are in those maps. The frame rates are all over the place. Some of that footage has correct lighting, some of it doesn't. All of that footage is olde."

Thank's Frankie, I cant wait to see the final product and the shield grenade and trip mine look bad ass.