People form impressions of others based on the smallest details. Perhaps when you read this message line you laughed at how improbable it would be to NOT get a job offer because your handshake was too strong or too weak.
Well, you're right, it doesn't work that way. However, it IS one of the details that people use to form an impression of you.
We've all been the recipient of those bone-crushing, gee-I'd-like-to-USE-that-hand-in-the-future handshakes. Think about the impression you form of the person who just delivered that blow. Do you consider them too aggressive? Inconsiderate? Powerful? Overcompensating? Overbearing? Are you likely to want to shake hands with them again? They could have a great personality and be a nice person, but nevertheless you form an impression of them from the handshake. That seemingly minor detail you then compile into your mental database to help you make an overall impression of that person.
What about those handshakes that are the typical limp fish? Or, those kind of "fingers-only" handshakes? They leave me with the impression that this person doesn't want to get too close to me because I'm somehow contaminated. I never get a warm, friendly, approachable impression of those types of people. I am personally more influenced by the weak vs. the ultra-strong handshake to the point where if I was interviewing them for a customer service position, I would scrutinize their work experiences and resume a little harder to assure myself I was hiring the correct person. All because of a handshake.
It is hard to describe a "perfect handshake", but I strongly recommend that you practice with a partner who will give you honest and direct feedback about what you are doing wrong. It might seem like a very minor detail, but when you're interviewing for the perfect position you don't want to take any chances.