If you top the plant it can serve a few functions, it will allow for more branches to grow from the cut area and split in a 'Y' shape where you top it, and it will allow a shorter, more bushy plant (avoiding tall and uneven canopies more effectively used with LST). Your main stalk cola will be eliminated into essentially two smaller cola branches, this can be good in avoiding mold as the main cola can sometime depending on your environment be more prone to mold due to the denseness of the main cola bud. The cutting from the topping itself can be used as a clone as well.
I have never actually topped and am in the process of my first grow, so wait for other people to respond. This info is an accumulation of researching topic from these same forums which I anticipate to do as to avoid mold on buds late in flowering and keep the plants bushier and shorter in stature. Also look at the bottom of this post to see relevant topics which automatically comes up and finds keywords your thread topic shares with existing threads, a neat and useful feature to do research fairly quickly even after you post.
The picture shows where you 'top', an where you 'fim', 'fim' is a similar technique to topping that results in more branches still off the cut area, doing however is apparently hit-or-miss. Again, people will follow this up with more detail and address what I said that is inaccurate or flat wrong. BTW, you said you had a picture?
EDIT: Perhaps this post is completely irrelevant if you are taking clones from the mother to put into veg, I thought it was to take clones from vegging plants to save and mark for after sexing, or that are female cloens already. I am not sure if people 'top' mother plants, good question.