What are the causes of Tetrology of Fallot(cyanotic spells)?
There is a "spasm" of the band of muscles just at the right ventricle narrowing. This leads to further decrease in blood flowing into the lungs and thus increased blue color as oxygen content of blood becomes very low. To compensate this, the patients breathe rapidly and try and take in as much oxygen into the lungs as possible.

There are several provoking factors that can precipitate cyanotic spell such as exercise, fever, diarrhea, dehydration etc. To compensate for these spells, children tend to squat to decrease blood supply to the right ventricle and thus decreasing right to left shunting of blood. Sometimes these spells may be life threatening.