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
REVOSHIFT SHIFTER | SHIMANO BIKE COMPONENT
SHIMANO BIKE SL-RS35 LEFT INDEX REVO SHIFT TWIST GRIP GEAR SHIFTER
How to replace the shift cable in a shimano SL-RS40 twist shifter - YouTube
Controller Left Shimano Revoshift SL-RS45-L+Cable New | eBay
Replacing Cable on Shimano SL-RS41 Grip Shifters : r/bikewrench