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 RevoShift SL-RS35-R Twist Shifter Right (6) - Walmart.com
How To Replace Shifter Cable On Shimano Revoshift Shifters - YouTube
SHIMANO - 6 speed Twist Grip Shifter (Right Shifter), Revoshift Sis In – Bike Blvd