positive down pressure on hitch

If you could let us know what is the purpose or reason for your request, it would be a little easier to advise, but in the case of a posthole digger there is a model sold here in Australia that has a additional hydraulic ram attached along the frame on top and back to a linkage onto the cross shaft which gives some downward pressure for use in very hard soils,
If you are using e.g. a chisel plough in very tough ground make sure that the points are not worn and the frame is level , sometimes it helps to fasten weights to the frame and also reducing the forward speed to an absolute minimum thus keeping the points at the correct angle this will create a greater downward draught then control the depth of the implement with the tractors draught control lever .
Please leave another posting if this isn"t the information that you was after.
Cheers Arthur from Tassy

Thanks Steve