In an article in Dog Sport Terry Simons asks "Where's the commitment?" He uses an analogy of driving on the street to explain. Usually when the light changes to yellow, we have time to decide whether to stop or accelerate through. But sometimes it is too late to make a choice - we are committed to driving through the intersection.
Your dog's commitment point is that moment when, with all four feet on theground, he makes an irreversible decision to take the obstacle successfully. Your verbal or physical cue should come at your dog's commitment point.
The effect if it doesn't can be explained using the driving analogy again. If you tell someone about the turn too soon, they make take the wrong turn or proceed hesitantly; if you tell them too late ... well we've all experienced that.
To find your dog's commitment point, Simons says to set up a circle or ellipse of jumps, making the pattern of jumps uneven so you have a bounce step, one stride, two strides, and even a-stride-and-a-half between jumps. Run your dog several times on this pattern, watching carefully where she takes off for the jump ahead of her. Make a mental and physical mark on this spot.
Your next step is to cue your dog to take the jump and then come to you. Do this at some time while running the circle. Make sure your dog is at a good speed and that she has taken several jumps prior to your call. Cue your dog at that point when you think she is going to take the jump successfully. Give your dog a verbal cue to come into you, and rotate your shoulders into the dog for a front cross. If your dog calls off the jump, your timing is too early. If your dog lands long and takes a couple of strides before turning into you, then you are probably late on your cue. If the bar comes down, then you were more than likely calling on top of the bar.
Videotaping the exercise will help you assess the commitment point even more clearly.