Centering the jacks in the middle of the repetition lever is an important step in the regulation process. Almost all actions will have at least a few jacks that are out of alignment. In some extreme cases, it can prevent escapement. The jack alignment tool from Schaff (R-28) makes the job easy.
On newer pianos, you can do this without removing the wippen. Older Steinways may require you to take the wippen out of the action. Slip the tool under the jack to support the side of the wippen in the direction that the jack should be traveled.
Lift the hammer out of the way, and gently tap the top of the jack with your handy hammer.
Or, with the wippen removed, hold the jack against the tool with one hand and tap with the other.
Your finished product should have the jack centered in the middle of the repetition lever window.