Hand signs give a physical placement for vocal pitches that helps with memory and singing on pitch. Solfège syllables (do re mi fa so la ti do) or (do re mi fa sol la si do) were created by Guido d'Arezzo, an Italian monk (990-1050), and hand signs by the Englishman Rev. John Spencer Curwen (1816-1880).
It is best to use both hands for brain coordination and balance. It also helps students to model teachers without left hand or right hand confusion. Some teachers use sol and si in place of so and ti.
Drawing adapted from www.classicsforkids.com