R Position

Hydraulic pressure is applied to the D inhibitor valve to engage the reverse selector with the third shaft reverse gear while the shift lever is in R.
Hydraulic pressure is applied to the 4th clutch, then the 4th clutch engages the third shaft 4th gear with the third shaft.
Mainshaft rotation is transmitted to the third shaft 4th gear via the mainshaft 4th gear splined with the mainshaft.
Rotation of the third shaft 4th gear is transmitted from the reverse selector hub that rotates with the third shaft to the third shaft reverse gear, via the reverse selector.
The third shaft reverse gear drives the mainshaft 3rd gear via the reverse idler gear.
The mainshaft 3rd gear drives the countershaft 2nd gear splined with the countershaft and the countershaft.
The rotation direction of the countershaft is changed by the reverse idler gear.
Power is transmitted from the countershaft to the final drive gear, which in turn drives the final driven gear.

Fig 1: R Position - Power Flow Diagram
Courtesy of AMERICAN HONDA MOTOR CO., INC.