Harmonic convergence:

It's more-or-less a diminished chord, passing between G and D major chords.  The curves show the harmonics-- multiples of the lowest frequency.

They converge at a high D#, probably perceived as a slightly dissonant color tone.  It's the 4th harmonic of the B, the mysterious 7th harmonic of the F, and (maybe irrelevantly? it's rather distant) the 18th of the bass note C#.

Also interesting is the F/F# at the top of the treble clef, where harmonics converge more sloppily.  Again, color sound, but also, a wider and more contoured energy train.