Do NOT fret these devices are for quick and easy centering requiring little or no thought by the operator. A skilled machinist would find little need of these unless they are on a productivity kick!
If you want more details on how it works I will copy the instruction sheet and try and describe the action. It comprises a DTI measuring the gap between the measuring plate held in the spindle being centred and another plate that is being pulled out of line by a measuring probe which is atached. The two plates are spring loaded to the neutra closed position that represents the aglined condition. The DTI is twisted to record the out of line error in the coresponding X or Y (assumed mill axis!) and the work positioned for zero deflection on the DTI in that axis. Repeated for the second axis. Bingo job centered to within 0.001 inch.
Hope this puts these devices in perspective. Worth the cost for repetative work particularly towards the end of a shift!