The sages of first and second century Israel used to open their lectures with a joke. This is because there are aspects of "Smallness" and "Greatness" in space, time, and soul (i.e. states of constriction and concealment, and states of expansion and revelation), and by means of a joke, one can rise from Smallness and cleave to Greatness.
Now, at a time of "Greatness", one can study and pray with fear and love. A person realizes that he is speaking to the exalted King, and can cling to him - there is no greater pleasure than this.
In a time of Smallness, a person must wage a great battle to come close to G‑d….
On the other hand, in a time of Smallness, a person must wage a great battle to come close to G‑d, although this is also the main source of reward, for a person must force himself [to serve G‑d]. Even when he cannot actualize his good intentions, nevertheless: "I have hidden Your word in my heart" (Psalms 119:11) - even when, outwardly, he seems far from G‑d, i.e. involved with material concerns, in his heart, he still remains close to Him.
Thus, the Sages said of Torah study and labor that "the effort expended at both of them keep sin out of mind" (Avot 2:2). Also, it is possible to perform mystical meditations (yichudim) in the stories that one person tells another. This is as the Maharsha explained, that when Nehemiah would talk to the king, he would have in mind to pray to G‑d (Chidushei Agadot, Rosh Hashanah 3b. See also Maharam m'Pano, Eser Ma'amarot, "Em Kol Chai," chap. 1:7). There are many similar examples.
When they are in a period of Smallness - and lack the fire to burn with an inner light, they are called embers….
With this, you can understand the statement "Warm yourself by the fire of the Sages" (Avot 2:10) - When the Sages are in a state of Greatness and are on fire with the light of the Torah and prayer, you should obviously warm yourself by their fire. However, "beware of their embers", meaning that when they are in a period of Smallness - and lack the fire to burn with an inner light, they are called "embers". The student who wants to learn from the proper approach to materialism may not realize that his teacher then fulfills the verse "I have hidden Your word in my heart" - which means to be engaged in physicality, but to have one's mind on inner meaning and spirituality. The student only learns from what he sees - that his teacher is involved in material pursuits - and will be punished. "For their bite is the bite of a fox, their sting is the sting of a scorpion [in Hebrew, 'akrav']." Meaning, the teacher draws the seven levels, each one composed of ten, totalling 70, the numerical value of ayin, the first letter of "akrav", and drawing them [related to the Hebrew root "krav"] back to their source. But the students do not realize this and think that their teacher is simply involved with material concerns. (Tzafnah Paneach, p. 31d)
I saw in the book Nivchar Peninim, in the name of a certain scholar, that a person should not draw to close [to a sage], for proximity can lead to distance.This may be the intention of the Mishna."Warm yourself by the fire of the Sages, but beware of their embers, lest you be burnt" (Avot 2:10): warm yourself only from a distance, and don't get too close to the burning embers, so that you do not get burnt.
It seems to me that I heard this in the name of my Master [the Baal Shem Tov].
[Translation and commentary by Eliezer Shore from "Sefer Baal Shem Tov on the Torah", Toldot Yaakov Yosef, p. 20c;
Reprinted by permission from www.baalshemtov.com]