“Why is the prayer of the righteous like a shovel ?
Just as a shovel [ehtehr/עתר] moves grain
from one place to another,
the prayer [ahtahr/עתר] of the righteous
turns G-d’s trait of anger to the trait of mercy.” 
Is this only true of the prayer of “the righteous?” Do the rabbis extol saints at the expense of ordinary people? No, I don’t think so. Elsewhere they say, “Whoever prays much will be answered.”  It’s prayer, not saints, that the rabbis extol.
Still, the analogy raises other questions.
Does G-d “change”? Can G-d be angry and punitive at one moment, then happy and loving at another? If so, it would contradict everything we’re told about G-d being changeless.
It would also mean that G-d’s reactions are determined by our actions – that we can “push G-d’s buttons.” Yet, when we tell even small children, “We choose our reactions, and can always choose better ones,” we mean: “No one else can ‘push your buttons’ unless you let them.” If we can choose to let someone “push our buttons” or not, then “kal v’homer” — how much more must it be true that G-d can choose!
To understand the rabbis, we must first realize that phrases like G-d’s “trait of anger” (Judgment/Din) or “trait of mercy” (Kindness/Hesed) don’t describe changes in G-d. Rather, they describe changing human perceptions of G-d. G-d, we’re taught, always gives the perfect, Loving response. As Rabbi Boruch Leff wrote:
“God through His judgment shows us that He cares about everything that we do…Through judgment, we are made aware that every little thing that we do makes a difference…We are significant and responsible. And responsibility is a tremendous cause for celebration.” 
We name that response as either “anger” or “mercy” depending on how it seems to us. In fact, the Talmud addresses this directly: Now (“in this world”), we say the brachah “Dayan ha-Emet” for seemingly bad news, and “ha-Tov v’ha-Mei’tiv” for seemingly good news,  but in a future time of higher, clearer awareness of G-d – we’ll say only “ha-Tov v’ha-Mei’tiv” for Good that’s done, because that’s how we’ll see everything that G-d does. 
Our own thinking creates the way G-d appears to us. Comparing praying with using a shovel (in Hebrew, a pun based on the similarity of the linguistic roots of both words), the rabbis are describing a change in our own thinking that comes about as a result of our repeated personal experiences of G-d’s Presence and Goodness in the course of praying.
As with all change, this is gradual; incremental -– like moving a large pile of grain with a shovel. Just as each “shovelful” moves some of the grain from one place to another, until it has all been moved, each sincere prayer — in which we experience Divine Love, even a little — can gradually change our thinking and our awareness of G-d from One who is “punishing” us, to One Who is always loving us. With each sincere prayer, G-d’s Goodness in our lives can become increasingly clear to us outside of prayer, too.
Rebbe Nachman of Breslav says the same thing:
“Even if many days and years pass and it seems as if you have accomplished nothing with your prayers and conversations with God, don’t give up! Every single word makes an impression.
‘Water wears away stone.’  It may seem that water dripping on hard stone could not make any impression, yet when water drips on stone continuously for many years, it can literally wear a hole in the stone. We actually see this.
Even if your heart is like stone and it seems that your words of prayer are making no impression at all, still, as the days and years pass, your heart of stone will also be softened. For: ‘Water wears away stone’.” 
The rabbis suggest even more: as G-d’s Goodness becomes more and more undeniable to us, more happiness, health, calmness and patience, even more material abundance, can fill us. That’s “G-d’s trait of ‘Mercy’.”
But this is only true when, in each prayer, we stop insisting that G-d do what we want, and instead make each prayer invoke what is Divinely known to be best for us. When first, “b’Yado af’kid ru’chi…”  – “I put my soul in [Your] Hands,” then “…v’lo ih’rah”  – “I don’t fear” follows.
In order not to fear, we must first sincerely place ourselves in the Presence of G-d and G-d’s Goodness, then let go of our problem. Fear disappears by itself, and we know only Divine Goodness filling every aspect of our lives.
 based on Yevamot 64a ; also Cohen, Rabbi A.; Everyman’s Talmud; p. 81
 Berachot 7b
 Leff, Rabbi Boruch; Judgment Means Love [on parshah T’tza’veh];
 Mishnah Brachot 9:5
 Pesachim 50a
 Ayov/Job 14: 19
 Rabbi Nathan of Nemirov; Kaplan, Rabbi Aryeh, trans.; Rosenfeld, Rabbi Zvi Aryeh, ed.; Rabbi Nachman’s Wisdom/Sichot [or Sichos] Haran; #234; no copyright date or publisher listed; p. 369
[This translation was done by the 1970’s; possibly earlier. There is a more recent edition ©1984 by the Breslov Research Center, but the text and pagination are the same.]
 from “Adon Olam“