Oster Power Threading Handbook
The pressure you exert on the carriage when starting the die head onto the pipe or bolt will influence the shape of the thread generated. The figure below shows the parts of the generated thread with which we are concerned.
If too much pressure is used to feed the die head onto the material the leading flanks of the first few turns (2 or 3) will be rough and grooved. If not enough pressure is used, the receding flanks will be rough and grooved. Either too much or too little pressure will cause the first few threads to be undersized resulting in a threaded piece that appears to be tapered on the end.
Proper starting pressures vary for different materials and vary by the diameter of the part. Generally more starting pressure is required for tougher or larger stock than for low strength or small materials.
With some experience an operator will get a "feel" for the proper starting pressure. It is not a difficult skill to master, just an important one.
It is a good idea to maintain the proper starting pressure for 3 complete threads, then relax and maintain just enough force so that the dies do not have to pull the carriage along.