The act of giving someone a "little something extra" to close a business deal or any other request or arrangement. This generally refers to a handjob, but could include anything up to and including all manner of sexual favours.
My wife wanted some new furniture, and I had said no, but when she closed the deal with a "Carlson Handshake", the next thing you know I am at the store buying a sofa with an extended warranty!
Charltons are one of the coolest people you'll ever meet as long as you stay on there good side. If you don't they could be the biggest asshole you'll ever meet... He'll stay by your side no matter what the situation is and he is a very good listener