Another exploded view:


It's a straightforward design. It's a one-way ratchet mechanism that is secured within a friction style shifter. Once dialed in properly, you overcome the friction to upshift, but when downshifting, the force needed is only to overcome the RD spring. The friction is left out thanks to the ratchet mechanism.
Hope that's clear.
There shouldn't be excessive binding when upshifting, the friction washers may need cleaning and adjusting.