Maniobra de Valsalva: ocurre cuando uno contiene la respiración y se esfuerza por defecar.

The "Modern" American Toilet Fixture  

Valsalva's maneuver
Increase of intrathoracic pressure by forcible exhalation against the closed (or significantly closed) glottis. The maneuver causes a trapping of blood in the great veins, preventing it from entering the chest and right atrium. When the breath is released, the intrathoracic pressure drops and the trapped blood is quickly propelled through the heart, producing an increase in the heart rate (tachycardia) and the blood pressure. Immediately after this event a reflex bradycardia ensues.  Many die from it.

"Valsalva's maneuver occurs when one holds one's breath and strains. Defecation or urination (many dies on the toilet seat), using the arm and upper trunk muscles to move up in bed, or strains during coughing, gagging or vomiting. The increased pressure, immediate tachycardia, and reflex bradycardia can bring about cardiac arrest invulnerable heart patients." -Above taken from a medical dictionary. 

They could have added "holding the breath and exerting" and it would have been even more clear. I believe holding your breath and exerting can cause a stroke and it most certainly over time makes speaking more difficult.

You may have done this on purpose as a child: Taken a lot of deep fast breaths and then stick your thumb in your mouth and blow without releasing any air. You get dizzy and pass out. Children can most often handle this, some adults will die from it. 

There are several breathing resistance training devices being sold via email that allege to be for sports enthusiasts. They are sold as something that can help strengthen the breathing by having people suck and blow in and out of them. Like a kazoo with an air-flowing in and out control. People can vary resistances and this resistance is supposed to strengthen the breathing.  I bought several varieties and tried them.  They made my breathing very tight and uncomfortable.  I just wonder how many sprinters or people that hold their breath and run, strain, get a stroke or just drop dead because of this. Sprinters may be more prone due to the fact that some hold their breath during an entire 100-meter short distance race. Sports-induced asthma is epidemic and I suspect these breathing resistance trainers are part of the cause.

I got this spam email, viewed the device, remembered I had bought similar ones several months ago, and visited a local respiratory therapist friend of mine.  He reminded me of the above Valsalva's Maneuver definition and suggested that with excessively increased pressure on the inhale, people can bust a "bulb" in their lungs as well as create "excessive intra-cranial pressure" possibly stroke causing pressure with this device. That was something used many years ago in respiratory therapy but shelved due to the aforementioned reasons. "COPD patients might receive benefits but others may be at risk". 

You can also cause the windpipe tube to constrict from the vacuum created and this may well weaken the windpipe and adversely affect the speaking and singing voice and invite apnea. Spasmodic dysphonia is one example and I suspect laryngitis and stuttering are as well. Some of the weight loss videos have exercises that create similar pressures. 

Constipation and Weight gain? 

Eating disorders may be the lack of healthy bowel habits among the vast majority of the population who can not distinguish a "full" rectum from irritation and discomfort of the bowel and therefore spend long periods on the toilet "bearing down" and creating diverticulosis and then diverticulitis with a faulty diet. Eating a low-fiber diet is one of the most likely associated causes.

The "MODERN"? toilet.  
Two years after a Dr. Sikirov's article on diverticulosis, the Journal of Medical Hypotheses (1990 Jul;32(3):231-3.) published another of his indictments of the "porcelain throne":

Cardio-vascular events at defecation are to a considerable degree the consequence of an unnatural (for a human being) seated defecation posture on a common toilet bowl or bed pan. The excessive straining expressed in intensively repeated Valsalva Maneuvers is needed for emptying the bowels in the sitting position. The Valsalva Maneuver adversely affecting the cardio-vascular system is the causative factor of defecation syncope and death. The cardio-vascular system of a healthy man withstands the intensive and repeated straining at defecation, while the compromised cardio-vascular system may fail, resulting in syncope or even death. The squatting defecation posture is associated with reduced amounts of straining and may prevent many of these tragic cases. 
One way for offsetting the tendency to strain while having a bowel movement. http://www.naturesplatform.com       

Straining or anything less than a gentle insistence of the need for defecation is too much force and heading towards Valsalva. A simple augmentation to keep your bowels open. I use it every night.

A PARTIAL SOLUTION
When most people sit on a typical western toilet, the upper legs are slanted downward. This position causes the large intestine to have too little pressure to help force excretion. Also the rectal muscles are somewhat compressed and not allowed to open as much as they should. Best position for defecation using a typical Western model toilet stool/bowl is sitting in a deeper squatting position so that one's  feet are flat but knees are several inches higher than the hips. This helps create a deeper squat, (babies are often birthed in the fields this way) and opens the crotch area and adds pressure to the belly/colon to help force evacuation.  While sitting on a western toilet you can use a foot stool that causes your feet to be raised 4-8 inches off the floor. This increases the opening of the anal sphincter and causes abdominal pressure to the colon helping it force evacuation. Loosen your bowels so as not to have to force.