Why people die on the toilet (version 0.1):
Although it doesn't get much press (for obvious reasons), a disproportionately high number of people (usually older men who are in high risk categories for heart disease) die on the toilet. This is at least partly due to problems some older men have with constipation. Although this causes them to generally spend longer on the toilet, that's not the problem. The problem is that people in this situation tend to "push" harder and longer than they should in what is effectively Valsalva's maneuver. This added pressure on the intestines and colon corresponds to an added pressure in the abdominal and thoraic cavities. The vena cava (major veins leading back to the heart) are normally "held open" by the "negative pressure" (lower than atmospheric... the same reason a hole in the chest wall (pneumothorax or hemothorax) can kill you so quickly). When the pressure increases it pinches the vena cava closed and the blood can no longer go back to the heart. Consequently, the barroreceptors which measure blood pressure on the arterial side of the heart, notice a drop in pressure (due to the fact that blood is no longer being pumped) and so they trigger a "sympathetic response" that causes blood pressure to spike. The combination of blood pressure spiking (which can lead to stroke or other problems) and lack of blood being pumped to the heart (which starves the heart for oxygen and eventually can trigger a heart attack and/or heart failure).

A similar effect can occur if a child tells another to hold their breath and then the child that is holding their breath (effectively performing Valsalva's maneuver on himself) is squeeezed around the chest by the other child (adding more pressure to the internal cavities). While this can lead to death, it generally just causes a child to pass out (because their cardiac tissue is much more resiliant and they are less prone to exacerbating factors like chronic high blood pressure or stroke).