Hand postures according to Torah:
We find in Chazal various hand positions which are encouraged or discouraged during prayer. The Poskim and Kabbalists record different postures which are always improper for one to perform due to Kabalistic reasons. Every limb of the body represents a different aspect of the Divine Sefiros, as a Jews body and soul were created in the image of G-d, and in the image of Primordial man, called Adam Kadmon. For this reason, the Kabbalists write that it is proper to look at one’s hands periodically and concentrate on the fact that the fingers correspond to Hashem’s Divine name of “A-H-V-H.” The following are the positions recorded in Poskim and Kabbalah which should always be avoided:
Grasping one’s fingers together: It is proper to beware not to fold the fingers of one’s hands together during times of peace, as by doing so one draws judgment upon himself [Heaven forefend, and worsens his Mazal]. This means that one should not interlock the [five] fingers of one hand with the [five] fingers of the other hand, like one who is breaking his hands when in pain. [This should be avoided during all times, even outside of prayer, and even during non-peaceful times. One is to especially be careful in this matter during the Ten days of Repentance, or when sick. One is to train himself to avoid doing so even unintentionally. If one does so even unintentionally, it is a sign from above that a decree has been made against him. In such a case, one is to perform an accounting of his soul and repent for his ways. One is to warn his wife and children against doing so. One is to protest against others when he sees them doing so. It is told that the Rebbe would make a point to mention this to people if he saw them grasping their fingers together in the above manner.]
Placing the hands behind the back: Based on Kabala, one is not to place his hands behind his back. This is because when one places the arms towards one’s back the arms can receive impurity from the external side, represented by one’s back, and become defiled from receiving blessing.
Lifting the hands: According to the Zohar, one may not lift his hands above his head for no reason. It is for this reason that when lifting the hands during the blessing of Al Netilas Yadayim in the morning after awakening, one is to begin the blessing right away, as soon as his hands are lifted. Likewise, those who stretch open their hands when saying Poseiach Es Yadecha is to beware not to lift the hands above one’s head.
 See Shabbos 10a; Rama 91/6; Michaber 95/3; Admur 91/6 and 95/4
 Shaar Hamitzvos Eikev “The right and left hand represent sublime spiritual powers and one is thus not to mix them”
 Shaar Hamitzvos Parshas Eikev; Ateres Zekeinim 46/2
 Admur 91/6 regarding position of hands during Shemoneh Esrei; Taz 95/3; Shaar Ruach Hakodesh 12b “One is to beware not to interlock the ten fingers of the right and left hand together”; Shaar Hamitzvos Parshas Eikev “It is forbidden for a person to interlock his fingers of his right and left hand”; Zohar Vayikra 24; 3/24 based on Rav Kahana in Shabbos ibid; Midrash Talpiyos Din p. 465; Meil Tzedaka 2/1; Or HaChama p. 23; Ateres Zekeinim 46/2; Zechor Asos Mareches Alef 82; Yesod Veshoresh Havoda Shaar Hakolel 19; Moed Lekol Chaiy 15/31; Nefesh Lekol Chaiy Mareches Alef 38; Mavor Yabok Sifsei Tzedek 14; Ben Ish Chaiy Pinchas 18; Yeshuos Chochma 33/17-18 in name of Nagid Mitzvah; Sefer Zechira; Kaf Hachaim 91/28; Piskeiy Teshuvos 95/5; See Leket Hakemach 91/12 and Imrei Shamaiy 2/180 who discuss that perhaps doing so today contains the prohibition of Chukos Hagoyim.
 Lit. “good”
 Zechor Asos ibid
 The reason: As the right and left hand represent sublime spiritual powers and one is thus not to mix them. [Shaar Hamitzvos Parshas Eikev]
 May one interlock some of the fingers together if he will not be interlocking all five? The source of this warning in the Zohar states that one is not to interlock his five fingers with five fingers. Likewise, in Shaar Ruach Hakodesh ten fingers is mentioned. However, in many of the other sources [including Shaar Hamitzvos and Admur ibid] no mention is made of the number of fingers and a general statement against interlocking the fingers is made. Vetzaruch Iyun. [See Piskeiy Teshuvos ibid footnote 42]
 Admur ibid; Aruch Hashulchan 91/7; Kaf Hachaim 91/28
 All sources ibid make no differentiation, and thus, although in Admur and Taz ibid this law is brought regarding positions of the hand during Shemoneh Esrei and regarding times of Shalom, it would apply during all times.
 Moed Lekol Chaiy ibid
 Mavor Yabok ibid
 Zohar ibid
 Zohar ibid; Or Hachama ibid “When the five levels of Gevura overpower the right it brings agreement to judge him”; Yesod Veshoresh Havoda Shaar Hakolel 19 “It is a sign that a great judgment has been decreed upon the person”; Nefesh Lekol Chaiy ibid “It shows that he has come out liable from the Heavenly courts for his sins”
 Nefesh Lekol Chaiy ibid
 Moed Lekol Chaiy ibid
 Or Hachama ibid “I saw elders protesting against those who do so”; Zechor Asos ibid;
 Zohar Vayeitzei 154; Shaar Hakavanos Inyan Birchas Hashachar regarding Netilas Yadayim Shacharis that the elbows should not extend towards one’s back; Ben Ish Chaiy Shmini Hakdama and Pinchas 18 based on Shaar Ruach Hakodesh that one is to always have his arms by his front; Sefer Hasichos 1944 p. 43 [p. 40 in Hebrew] that the Alter Rebbe was careful in this matter due to the above warning based on Kabala; Tehilas Chaim 1/241 in name of Divrei Chana in name of Divrei Chaim
Other opinions: Some Poskim rule it is permitted to hold the hands behind one’s back and it is even exemplary to do so during prayer. [Ran on Shabbos 10a in name of Rabbeini Chananel that one is to fold his hands behind his back during prayer like a slave before his master; Beis Yosef 95 in name of Mahariy Abuhav] Accordingly, some Poskim are Milameid on those who hold their hands behind their back. [Tzitz Eliezer 9/4; Piskeiy Teshuvos 95 footnote 32]
 Ben Ish Chaiy Toldos 4; Vayigash 12; Torah Leshma 31; See Piskeiy Teshuvos 95/4