Watch the Technical Mix online guitar lesson by Brad Carlton from Chord Studies: Open String Chords Vol. 2
You could view this progression as a I-IV progression in the key of E but we'll discuss how when you go to that IV chord you are technically in another key. Our first chord is the Esus2sus4, and then our second finger will act as a pivot and our first finger as a guide finger down to the E chord. We are actually going to now apply this same movement to the A chord. You'll notice in the Asus2sus4 we've got a D note in there, which wouldn't work in the key of E major so we are actually in E mixolydian based out of A major.