“…when I stretch forth My hand over Egypt…” 
Later: “And G-d said to Mosheh: ‘…Take your rod and stretch forth your hand over the waters of Egypt’…” 
Why was it necessary for Mosheh to “stretch forth” his hand (or to have Aharon do so)?
Because by doing this, Mosheh was emulating G-d: “…when I stretch forth My hand over Egypt…”
G-d’s “hand,” of course, is a metaphor for G-d’s “power.” In the same way, Mosheh’s or Aharon’s “hands” do nothing at all themselves. Stretching them out represents, even invokes, G-d’s activity.
In fact, for each of the plagues, Mosheh (or Aharon) must first “stretch forth” a hand with the rod in it or, in some other way, do something to initiate or end the process:
1. G-d said to Mosheh, “Say to Aharon: stretch forth your hand with your rod…and cause frogs to come up…” 
2. “Say to Aharon: ‘Stretch forth your rod and strike the dust, that it become gnats’…” 
3. “…G-d did according to Mosheh’s word…” 
(In this case, the same principle, employed verbally, is demonstrated for ending the plague.)
4. (“…Tell Pharaoh): ‘There will be a very serious murrain’…” 
(In this case, the principle, employed verbally, begins the plague.)
5. “…take handfuls of soot of the furnace and throw it towards heaven…” 
(The dust will become boils, which can have a “burning” feeling. Again, the process is begun with an affirmative declaration or action).
6. “Stretch forth your hand so that there will be hail…” 
7. “Stretch forth your hand over the land of Egypt for the locusts…” 
8. “Stretch forth your hand toward heaven and there will be darkness…” 
9. Mosheh said [to Pharaoh] “G-d says: ‘About midnight I will will go out into the midst of Egypt; and all the first-born of Egypt will die…” 
(As earlier, the process is initiated by a verbal declaration).
Each of the plagues, then, begins with an act by Mosheh (or Aharon, representing Mosheh) that results in a display of Divine Creativity.
This is true of G-d’s Creative process, too. G-d first declares it, and then it’s so: “G-d said, ‘Let there be light,’ and there was light” (Ber./Gen. 1:3).
We observe, then, that Divine Creativity must be preceded by a statement or action that first specifies and affirms it.
“Divine Creativity” includes “spiritual” or “mental” healing. Such healing begins with a statement or action that specifies and affirms it.
From this, we learn something about prayer: Prayer-results (also called “demonstrations”) follow the direction of our declarations, even if made in thought rather than verbally spoken. As with Mosheh, G-d — the Divine, Holy Mind that creates, sustains, fills and guides all — “does” according to our “word.”
Yeshu, whom the world calls ‘Jesus,” taught a similar prayer-principle: “…if anyone says to this mountain, ‘Go, throw yourself into the sea,’ and…believes that what he says will happen, it will be done for him.” 
Jewish tradition has declined to accept Yeshu as Moshiach or as other than human. But we can still recognize the validity of what Yeshu taught about prayer — which had been demonstrated earlier in Torah (and elsewhere in TaNaCh), too — without compromising our other positions.
Note that “believe” here refers to: believing that what you declare in prayer will actually happen, which also implies believing that G-d can do anything. This is later echoed in the Epistle (letter) of Ya’akov (James): “But when [one] asks, he must believe and not doubt…” “Believe” here again means “believe that what you declare will happen…;”  and likewise — “…the prayer offered in faith will make the sick person well…,” where “faith” means the same as “belief.” 
Whether we know it or not, we’re always praying. We’re always creating the features of our lives, based on our words and thoughts. We’re never “more” or “less” in G-d’s Presence. “Prayer” is the time we take to give that truth more, rather than less, of our attention. But we’re always thinking, and our thoughts are always producing effects and results — because the Divine Presence is always filling and surrounding us.
Thus, when an anxious hasid whose child was direly ill came to the Tzemach Tzedek for prayer, his rebbe told him, “Tracht gut vet sein gut” — “Think ‘good’ and it will be ‘good’.”  The child was healed even before the hasid got back home, because the hasid had changed his thinking! Note here, too, that neither the rebbe nor the hasid “prayed” in any formal sense. The hasid changed his thinking; perhaps it’s also implied that the rebbe quietly “thought good” for the hasid’s child, too. Informally — we’re always “praying.”
At every moment, then, we’re declaring in thought, word, and action how we want G-d to manifest in our lives.