Tatum used a special device to elongate his descending pentatonic runs, to sustain them over longer groups of bars. In addition to inserting descending chromatic connective tones, he also inserted some ascending skips (usually fourths, fifths, or sixths), followed by a descending skip landing a whole-step below any of the tones of the pentatonic run he was using. In this way, he was able to "gain" some ground, and was then ready to play another portion of the descending pentatonic run.
— The Right Hand According to Tatum: A Guide to Tatum's Improvisational Techniques Plus 10 Transcribed Piano Solos
by Riccardo Scivales and Art Tatum