Watch the Back and Forth online guitar lesson by Jon Finn from 30 Melodic Modal Rock Licks You MUST Know
"Back and Forth" is in F# Phrygian. This lick uses lots of half step and whole step bends with releases. It's called "Back and Forth" because you hit back and forth the key notes of this lick. Think of this lick playing in F# minor scale, but with a lowered second degree. The lowered second over the F#m chord is what gives the Phrygian scale it's character.
It opens with a C# bending up to a D, then back to C#. Then, bend an F# up to a G and back, which is the b2 of the Phrygian scale. Then, back to the C# in a lower position to finish the phrase.
The second half is over a G chord. On the last note, keep the vibrato nice and smooth as you sustain the note. It will take practice and patience, but be mindful of how it sounds. Check the notation and tablature to get the finer details. Practice with the backing track, then invent your own version of this lick!