I want to share an interesting Dvar Torah from Rav Mordechai Schifman of Emek Academy in Los Angeles. Have a great Shabbos and a Kesiva v'chasima tova!!
"Hashem will command the blessing for you in your storehouses..."(28:8)
The word "asamecha" - "your storehouses" is derived from the word "samui" - "hidden". Therefore, the Talmud extrapolates from this verse that blessing is only bestowed upon that which is hidden from the eye. Once the eye has dominion over an item, blessing can no longer be conferred upon it. This is the source for the concept known as "ayin hara" - "evil eye", the ability to wish evil upon oneself or someone else. What are the mechanics of ayin hara?
The Talmud relates that if a person wishes to ward off an ayin hara, he should enclose his left thumb within his right palm, and his right thumb within his left palm, and proclaim the following: "I am a descendant of Yosef." Is this a mystical procedure or does it have a logical basis?
My Rosh Yeshiva, HaRav Yochanan Zweig Shlit"a, offers the following insight: The world has been given to man for his use. As long as man realizes that the world is under Hashem's dominion, he benefits from Hashem's protection. When man attempts to seize control from Hashem, he loses his connection to the source of blessing which offered him protection. When a person sees something he desires, he attempts to take possession of it visually. The visual control that a person exerts over an item can impact on it negatively. As long as a person is in Hashem's protective custody, he cannot be affected by another person's vision. When man flaunts his possessions, he attempts to grasp dominion from Hashem. Even when man does not consciously flaunt his possessions, but nevertheless displays his good fortune in a manner which draws the attention of others, he subconsciously takes credit for that which he has. Consequently, he loses protection, and becomes susceptible to the eye of others controlling him.
The Talmud teaches that Yosef's descendants are impervious to the evil eye, for Yosef did not attempt to grasp that which did not belong to him, even visually, i.e. Potiphar's wife. Therefore, the eye cannot control him. If a person is afraid of being attacked by an ayin hara, the Talmud instructs that he should grasp his thumbs. The reason for this procedure is as follows: The hand is the body's controlling and grasping limb, and the finger which dominates these movements is the thumb. By grasping his own thumbs, a person is immobilizing his ability to grasp and control. Therefore, by following this procedure, in conjunction with the statement that he is from Yosef, who refused to grasp or control that which did not belong to him, he proclaims that he is not a person who grasps or controls that which does not belong to him. Thus, he hopes to be afforded the necessary protection from ayin hara.