According to the Zohar, the level of atonement achieved through the sincere recital of Tachanun cannot be accomplished at any other time or in any other way in this world. In earlier days, Tachanun was said with one's face pressed to the ground and one's body stretched out in total submission to God (Gemara Megillah 22b; Rambam; Tur). Our custom is that we do not bow fully when reciting Tachanun, but place our head on our arm as a reminiscence of bowing.

According to many Poskim, commentators, “falling” for Tachanun is only performed when one is davening in a place where there is a Sefer Torah (Rokayach, Maharil, Agur, quoted in Beis Yosef and Darchei Moshe 131). Because the pasuk tells us that Yehoshua fell on his face in the presence of the Aron, Ark (see Yehoshua 7:6), these poskim assume that the concept of “falling” Tachanun requires the Aron or a Sefer Torah except in the city of Jerusalem where the city itself is considered “holy.” Tachanun is the only part of davening where the Hazzan does not stand. Since the entire purpose of the Tachanun is to recite a prayer while one is bowing, the Hazzan also “falls” Tachanun.

The accepted Ashkenazic practice is to lean on the left side when not wearing tefillin, and on the right side when wearing tefillin so as not to lean on the tefillin (Darchei Moshe and Rama). A left-handed person should always recite Tachanun while leaning on his left side.

Ashkenazim recite Chapter 6 of Tehillim, Psalms, while “falling Tachanun.” After this, they say the prayer “Shomer Yisrael” while still sitting, and then they begin the prayer “Va'anachnu Lo Neida.” The first three words, “Va'anachnu lo neida… We do not know,” are recited sitting, after which one stands up to recite the rest of the prayer. In addition, we say the first five words of this prayer aloud “Va'anachnu lo neida, mah na’aseh….” We do not know- what shall we do? This practice is observed in order to emphasize that we have attempted to pray in every way. We davened Shmoneh Esrei while standing, Tachanun while bowing, and other prayers while sitting down. Finally we declare, “Va'anachnu lo neida,” “We do not know!” We have tried every method of tefillah, prayer that we are commanded and we are unaware of any other more, “Mah Na’aseh,” What (more) can we do? (Magen Avraham 131:4).

Moshe Rabbeinu ascended Mount Sinai to receive the second set of Tablets on a Thursday, and returned with them forty days later on a Monday. Hashem's decision to give Moshe these Tablets clearly implied that the Jewish people were forgiven for the sin of the Golden Calf. As a result, Monday and Thursday became etched into the calendar as days of repentance and Divine favor for the Jewish people. This is why these days are chosen for fasting and special prayers. Therefore we preface Tachanun with “Vehu Rachum” a lengthy prayer, recited while standing, on Monday and Thursday mornings.

Though one should recite Tachanun when davening alone, it is preferable for it to be said together with a minyan (Rambam; Tur). Therefore someone who finished Vehu Rachum before the tzibur should wait in order to respond “Sh’ma yisrael … to the Hazzan and begin Tachanun together with them (Be'er Heiteiv 134:1). Similarly, if davening with a “Heicha Kedusha” not reciting the full repetition of Shmoneh Esrei, one should wait to say Tachanun together with the minyan.

There are many dates or special occasions when the accepted practice is to skip Tachanun, however space does not allow us to explain the reasons for each of these customs.

The Rambam writes that the most important aspect of Tachanun is to make personal requests. He pointedly states that there is no limit to the number of personal requests one may say. Many follow this practice. It is essential to appreciate that Tachanun is a time when one can include private, personal tefillot and sincerely beg God for whatever we lack. So let’s not skip Tachanun, let’s take advantage of it.