Pinching is usually taking out the youngest leaves on top of a plant. You can use fingers to "pinch" out the shoot tips so the plant would branch. This would result in a more compact plant. Deadheading, on the other hand, is just picking off spent flowers and throwing them away. This can make the plant look more tidy and prevents rot on the dead flowers. Deadheading may also encourage more flowers since energy would not be put into making seeds. I hope this helps. Sorry I don't have a picture...maybe someone else can post:)

When dead heading- follow the flower all the way back to the main stem and snip it there. (a sharp fingernail or small pruners). When I first had petunias, I would just snip of the flower then ended up with long stems full of dead little sticks. For a lot of plants follow the flower back to the next leaf and snip it off, don't leave anything above the new leaf. I hope you understand what I am trying to explain. It is so simple, but for years I spent all of my time dead heading and wasn't taking it back far enough until I saw another gardener show me how!

Will deadheading a petunia this way prevent it from getting leggy? I've always snipped the dead bloom off right where the bloom meets its stem and the plant always looked scrawny anyway.

With petunias you have to pinch the whole stem back to get new branches to grow. If you look at the plant, you will see new stems ready to pop at the leaf nodes along the bottom of the stem. Cut the leggy stems back to these fresh baby stems.

I didn't know this either.
I always just pull off the dead flower ususally leaving the stem!
Thank you for explaining.