1 to 1 action rear derailers are suggested for grip twist shifters. Some derailers do not move good enough with the shifter.
From here: Currently there are multiple conventions for the relationship between shifter travel and rear derailleur travel, known as actuation ratios. The ratios, when given, are nominal, and do not represent an exact ratio.[citation needed]
One convention, used by Shimano, is one-to-two (1:2). A unit of cable moved in causes about twice as much movement of the derailleur.
Another convention, used by SRAM mountain bike rear derailleurs, is one-to-one (1:1). A unit of cable moved in the shifter causes about an equal amount to be moved in the derailleur. SRAM claims that this makes their systems more robust: more accepting of contamination.[11]
Exact Actuation, used by SRAM road bike rear derailleurs, similar but different from the mountain ratio.
Campagnolo convention.
Suntour's convention.
Shifters employing one convention are generally not compatible with derailleurs employing the other, although exceptions exist.[12
Derailleur gears - Wikipedia, the free encyclopedia