The way it works here (Ace) is the conventional simple space riff is in the key of G (D mixolydian) taking you away from the last series of chorus lyrics.
Then an abrupt change in the feel occurs, an instantaneous shift to the key of C. (D dorian)
Garcia, upon this change, drops back for an instant, letting the band set up the backdrop of D dorian, and then he drifs up into his high note lead and stays firmly in the grip of D dorian.
Within the jam, he intersperses a little chromaticism, but just to get him to different elements that are possible against the D dorian mode. It is an extremely fun jam to execute for the high note lead lines because they are so dramatic against the dorian feel. Moreover, technique here allows for all sorts of persuasion.
Hitting a high note repeatedly and dragging your finger to the next lower note fret, over and over, emotes an extremely powerful falling into sadness sensation. Also used separately here is the combination of accurately timed picking of a note with a hammering of a note two frets higher, that produces an offsetting upward cascade, allowing the picking to take on atmospherics of utterly fast, as though impossible, repetition of tight note singles and pairs. He works it to perfection, never letting the listener come up for air, removing all ofthe air in the cyclonic vacuum of the technique, as though suffocating the listener to the point of hallucination.
Finally, he comes out of the D dorian mode at the very end, a very dramatic change, the kind of change where Garcia brings everything down to an essence, or better, a quiescence, and changes the tone, and mode, on a single note, a G string high eleventh fret F#.
Everything shifts back again to the key of G. (D mixolydian)
So the jam take away, jam proper, and return involve using a key modulation of simply G to C, and then back to G again. But because the emphasis is D mixolydian, to D dorian, and then back, the result is one of the most finely articulated examples of the Dead's depth, producing one of the best Grateful Dead studio jams ever recorded.