Why does operating the rudders in the engine slipstream offer improved rudder authority at low speed?
A propeller accelerates the air flowing through it in order to create thrust. This acceleration is highest at low speed.

The acceleration will increase the dynamic pressure at the tail if it is in the slipstream. A higher dynamic pressure will cause a proportionally higher aerodynamic force. A H-tail will, therefore, only be in the slipstream if two engines are mounted ahead of it, like in two- or four-engined designs with wing-mounted propellers. Also, by placing the area of the vertical low, a H-tail offers better dynamic flying characteristics.