The plunger is what launches the ball. Some people use just a simple button to launch the ball, others pay hundreds of dollars for complicated systems for dealing with the ball. Whatever you choose will be up to you and your budget. There are premade options and plenty of DIY solutions.
There are several places that sell a digital plunger which can be purchased separately or with a nudge card as a combo. Some people have made their own using old parts, or having a bolt with a washer on the end of it that activates a leaf switch.
List of places to buy a kit:
Note that the plunger does not work without the nudge card, so make sure if you are going to buy it separately, you have their nudge card already.
NanoTech
Zebsboards
OZstick
There are many creative ideas on how to convert the plunger to digital input. Some are not any better than just adding a button. Others are very creative. There are some like the optical solution described in some faqs that are not options anymore because parts are no longer available. Whatever you choose is up to you.