This picture is from www.longhaircareforum.com It is an example of new growth -> relaxed hair -> underprocessed relaxed hair -> relaxed hair. The underprocessed hair looks slightly looser than the new growth at the scalp. Do you see the underprocessed area in the picture?
It is like a tree's rings. The farther out that the underprocessed area is then count a half inch a month to figure out when the underprocessing took place. Based on this picture, about eight months ago, this person's relaxer did not take very well. I estimated the underprocessed area was four inches down from the scalp so 4 times a half inch/month equals about 8 months.
What can cause underprocessing? 1) Too much product (i.e. oil, grease, buildup, excessive base) is on the hair. 2) The room is too cold or client is too cold for the relaxer to process within a timely manner. 3) Client is on their menstrual cycle and their body's chemical balance is off. 4) Relaxer is not strong enough for the hair type and curl pattern. 5) Relaxer is rinsed out early due to burning or concerns about overprocessing in other sections of kinky hair. 6) Client drank excessive amounts of juice or other acidic drinks prior coming to hair appointment. 7) Client waited over 12 weeks between relaxer touchup services. (excessive new growth)
Relaxers typically have a pH between 11 and 13.5. Anything on the hair that "dilutes" the relaxer like excessive base or product buildup can interfere with the relaxer processing in a timely manner. Normal body heat is also needed for the relaxer to process. Please note: Do not put a client under a platform dryer with relaxer in their hair because the dryer heat is too excessive. Some alternative relaxers may instruct you to do otherwise; however, the pH in the alternative relaxers may be a pH of 10 which may be too low to get straight results without adding additional heat.
So for whatever reason(s) you find that you have under-processed/underprocessed hair, you may suffer hair breakage at your "snap points" or where the straight hair meets the underprocessed hair.
This picture came from www.mylonghairjournal.blogspot.com It is highlighting how combing naturally kinky hair too roughly can cause breakage. It also gives you a good mental picture of how underprocessed hair takes more resistance than straighter hair. Your blow-dry stage could cause breakage at the line of demarcation connecting the underprocessed and the straight hair.
How to solve the underprocessing? 1) Run the relaxer through the underprocessed areas. (i.e. Corrective relaxing) 2) Roller set the hair so that it will not be so noticable. 3) Eventually trim it away.
If your hair is fine, overcurly, and underprocessed - then corrective relaxing may be overkill, but to do nothing could cause breakage. It is a tough call. Medium to coarse hair can survive a corrective relaxer. If you have a loose curl pattern, then you are probably used to some underprocessing, and the relaxer is just used for some styling control versus straightness. I have found that some wavy hair will not relax straight, but the relaxer application helps control frizz. It is a judgemental call. Every person has to make a decision based on their hair type.
This "underprocessing" is also considered texturizing. Consistent relaxing is important to keep your hair strands uniform. Ideal touch-ups should fall between 5 and 10 weeks for curly to kinky-curly hair. Wavy hair can touchup around every 3 to 5 months.