Pacing is when you enter the other persons model of the world on their terms. Like walking beside them at their pace. Once you have paced another person, established rapport and shown that you understand where they are coming from then you can lead them.
Leading is when you use the influence that you have built up from pacing. For example, when you scratch your nose, they go on to scratch theirs.
A common approach is to start with three pacing statements, followed by one leading suggestion. For example: “And as you sit there (P), your eyes blinking (P) and your breathing slow (P), you can begin to wonder just how much of that ‘calm peace’ you can continue to feel now (L).”
However, it can be more subtle than this. The following description is provided by Doug O’Brien:
‘I used this pattern frequently when I used to work at New York’s Columbia-Presbyterian Hospital’s Department of Complementary Medicine. I was there as a Hypnotherapist to assist people in feeling better about their operations, facilitate healing and recovery, develop new, healthier patterns of living (e.g. becoming a non-smoker) and to help manage pain. Often I would walk into the patient’s room and say, “Hello, my name is Doug (easily verifiable due to the name tag on my lapel), I’m with the Department of Surgery (also on the tag), and I’m here to help you with your pain.” The two verifiably true statements set up the following statement to be accepted as true, too.’
Anthony Jacquin does a great job of explaining pacing and leading in this video.
