Originally Posted by
tFUnK
To be fair I've had inline adjuster that were easy to adjust with one hand, and others that required the other hand to hold the non-rotating end in place. Whether this discrepancy is a product of design intent or matter of wear over time, I can't say.
No one makes a barrel adjuster where you are supposed to hang on to a piece that disappears entirely into the body of the adjuster. The ones that take two hands aren't working right.
The side that recesses has something in it that is supposed to grip the housing ferrule - usually something rubbery. If that recessed part is turning, it is either because the mechanism friction is too great or the ferrule traction too low due to grime or wear. If the mechanism is bottomed out and you keep turning it, that will damage the rubber gripper over time.
There are also barrel adjusters designed to into cable stops or shifter bodies. Same principle - you can see the rubbery grommet on the narrow end that keep them from rotating:
