Watch the Math Rock Shuffle One online guitar lesson by Angus Clark from Hard Rock Survival Guide: Rhythm
This exercise is meant to expose you to how time signatures can be shifted when using riff-based material. The chording is not very complicated, nor is the riff. Just focus on playing the part as precisely as possible, so that you lock up with the bass and drums. You may find the shift in time signature disconcerting at first, but once you have the part learned, it'll all fall into place. I am using the E minor pentatonic scale at the fifth position during the time signature shift section. Please be sure you have this fingering learned.