The three-hand effect is a means of playing on the piano with only two hands, but producing the impression that one is using three hands. Typically this effect is produced by keeping the melody in the middle register, with accompanying arpeggios in the treble and bass registers.
Excerpt from Thalberg's Moïse fantasy illustrating the "three-hand" effect, and indicating use of piano pedals
Sigismond Thalberg was an Austrian composer and one of the most distinguished virtuoso pianists of the 19th century.
Lithograph by Josef Kriehuber, 1841
Thalberg in 1826
Thalberg in 1836
Francesca Lablache, Thalberg's wife