My daughter's Trek came with a 6-speed Shimano Revoshift SIS Index. It's a new bike. She can shift to higher gears easily but does not have the grip strength to shift back
Shimano SL-RS36 Revoshift 6 speed Shifter Set, left and right - Good Cycles
Shimano Tourney Revoshift SL-RS35 Shifter - 3S - Left