As we mentioned earlier, one who emits seed, as well as one who has a “zivah” type of emission, is only impure once the emission leaves his body. What is the source for this law? The Torah said about him, “Any man who will have a discharge from his flesh...” – from his flesh and not in his flesh, to tell us that it has to be out “from his flesh.”
Thus, if a Kohen was eating his holy portion (terumah), and he felt his limbs trembling, he grasps the male organ, to prevent semen from exiting his body, and quickly swallows the terumah food. But how could he do it? We learned that one should not touch his member!? Abaye answered, “He grasps it with a thick cloth, as it does not cause additional arousal.” Rava said, “Even thin cloth, since no additional semen will be emitted on account of it,” and those with whom it will are anyway a minority.
On the subject of “trembling of the limbs,” Shmuel said: “Any semen that is emitted without being felt throughout one's body does not convey ritual impurity.” Another version of what Shmuel said, “Any seed that does not shoot forth like an arrow cannot fertilize.” According to the second version, semen that exits without a feeling cannot fertilize but still produces impurity.
Art: German School - A seated Peasant eating a Meal