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-RS35,7 Speed Bike Right Shift Lever for Mountain Bicycle
Shimano Tourney SL-RS35-L RevoShift 3 Speed Left Twist Shifter With Gear Cable | eBay
Shimano Revo 7 Speed Twist Shifter – Infinity Cycle Works
Shimano 7 Speed Sis Index Revo Shift Bike Bicycle Cycle Shifter Derailleur | eBay