A:  Well, being that you are a woman, I am going to use your extensive laundry experience to illustrate this :)). You know what towels look like when you pull them out of the washer (before you put them in the dryer), the whole towel is still there but it looks so much smaller and all clamped up? Well, that is your clamped fin betta.

Or let us do this. I want you to stand up, with feet apart and raise your arms in a V shape and stretch them out as far as they can. This is your healthy betta with fins that are open and spread out. Now I want you to put your feet together and bring your arms down along your body. This is your clamped fin betta. The fins are still there, they just are not open. They are in a closed position, so the betta looks as small as possible.

OK, so you are like: "Laundry, Gymnastics, this is all fine and dandy but I still can't visualize it.". Very well then, let us use the old "a picture is worth a thousand gyrations" moto :)).  

So here is two photos of a female, which will clearly show the clamped fin look. Please take a moment to click on each photo to see a bigger version of it.:

open fins position - all fins are spread out and looking wide.

clamped fin position - all fins are now tightly hugging the body of the betta making the betta look much thinner, smaller.

Bettas will go into a clamped fin position for a couple of reasons:

• sickness. A sick betta may have clamped fins. This is especially true when the betta has velvet or other external parasites. This is also often true with bacterial outbreaks, but not always the case. Hence some sick fish may not exhibit clamped fins. Poor water quality or untreated tap water will cause bettas to clamp their fins. So check your water parameters and make sure they are OK!

• fear. When being manipulated (netted, water changes, released in a new tank) bettas will clamp their fins temporarily. My feeling is that the manipulation makes them nervous and when bettas feel nervous their reaction is to make themselves as inconspicuous as possible. By clamping their fins, bettas look smaller and in a hostile environment, being smaller always works in your favor, because you are harder to spot, and may go unnoticed (hence not get eaten by a predator). 

Now if your betta has clamped fins AND is not eating, it is a sign that he is sick. Please see my disease page to try to determine what may be wrong with him. If he acts normal and just does not eat a lot from time to time I wouldn't worry too much about it. Some vitamins may be good for him, I recommend Vitamix by Aquatronics.