This particular musician is improvising in the Stride style of the Ragtime genre of Jazz.
“Harlem Stride Piano, Stride Piano, or just Stride, is a jazz piano style that evolved partially from ragtime. The left hand may play a four-beat pulse with a single bass note, octave, seventh or tenth interval on the first and third beats, and a chord on the second and fourth beats… Unlike earlier “St. Louis” pianists, stride players’ left hand often leapt greater distances on the keyboard, and they played faster and improvised.” — Wikipedia
“Flipping difficult.” — Susana, resident Geekosystem piano nerd
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