Choking occurs when a foreign body obstructs the trachea. In nuclear physics, a poison is a substance that obstructs or inhibits a nuclear reaction. Repressor proteins bound to the operator sequence physically obstructs the RNA polymerase enzyme, preventing transcription. The hematoma mechanically obstructs blood flow from the perichondrium to the avascular cartilage. Later he deliberately obstructs the movement of mercenaries by cutting the trees to distract them. Counter-mobility obstructs enemy troops by putting up obstacles. This debris obstructs the flow of filtrate, which results in reduced output of urine. Varaha agrees to protect Brahma's sacrifice and to destroy any evil that obstructs it. It, too, obstructs the foramen of Monro. The MBT is often cumbersome in traffic and frequently obstructs the normal flow of traffic. Other safety features include a firing pin block which mechanically obstructs the firing pin. This obstructs opposing checkers that are behind the prime. When Rahim obstructs him, he arrests him too. This occurs when new information is processed that obstructs the retrieval of old information. Thus the equatorial phenyl group on phosphorus obstructs the "Si"-face, resulting in excellent enantioselectivity.