Overdrive is earned by hitting the overdrive notes section perfectly with no overstrums, overdrive notes glow white.
In order to fill the bar you will need to hit 4 sections perfectly (There are sometimes double earning sections that are denoted by the edges of highway being glowing just like the notes.)
Once the bar is full avoid pressing
, tilting your guitar, tapping
/taping the mic during breaks in the song or free drumming and hitting green on the drums. When the song ends with the bar full the achievement is yours.