Without vowel markings the Hebrew word for "almond" and one of their words for "watch" look the same, שקד. With the vowel markings, almond is שָׁקֵ֖ד (šā-qêḏ), and "watch" is שֹׁקֵ֥ד (šō-qêḏ). In the days prior to vowel markings, those who read the text would have had to have heard it read to them with the distinctive vowel sounds, otherwise the difference would have been lost over time.
The use of these two words could be a pun that Jeremiah's audience would have readily understood, but for us, many, many centuries from the time, the pun, if that's what it was, seems very obscure.
שָׁקֵ֖ד (šā-qêḏ) and שֹׁקֵ֥ד (šō-qêḏ)
Moreover the word of the LORD came unto me, saying, Jeremiah, what seest thou? And I said, I see a rod of an almond šā-qêḏ tree. Then said the LORD unto me, Thou hast well seen: for I will hasten šō-qêḏ my word to perform it.
-- Jeremiah 1:11-12 (KJV)
A rod is used for discipline, which is how God intends to use it.
This particular rod, though, is an almond branch. Gesenius suggests the significance of the almond branch is connected to the fact that almond trees are the first to wake up after winter. They are "watching" trees, i.e. trees that watch for the approach of spring. So, the branch will be a "watching" rod.
Okay, if שֹׁקֵ֥ד (šō-qêḏ) means "watch", why has the KJV given "hasten". The only reason I can imagine is that the almond tree doesn't just watch for the signs of spring, it is also eager for spring to hurry along. Gesenius suggests there is a hurry-up sense to the word, as well.
This idea will seem a little more plausible when the other uses of שֹׁקֵ֥ד (šō-qêḏ) in Jeremiah are investigated.
Other uses of שֹׁקֵ֥ד (šō-qêḏ)
Wherefore a lion out of the forest shall slay them, and a wolf of the evenings shall spoil them, a leopard shall watch šō-qêḏ over their cities: every one that goeth out thence shall be torn in pieces: because their transgressions are many, and their backslidings are increased.
-- Jeremiah 5:6 (KJV)
Here שֹׁקֵ֥ד (šō-qêḏ) is used to depict a leopard watching and waiting for the gates to open, with the sense of hurry-up-and-open, so that when the people venture forth they can be pounced upon and torn to pieces.
Behold, I will watch šō-qêḏ over them for evil, and not for good: and all the men of Judah that are in the land of Egypt shall be consumed by the sword and by the famine, until there be an end of them.
-- Jeremiah 44:27 (KJV)
A group of God's people have defied His instructions and run to Egypt for protection (Jeremiah 42,43), and God tells them through Jeremiah, that He will bring Egypt's enemies down upon that nation, and those of Judah who have no heart to return to Jerusalem will be consumed in the process.
In the past God had put a hedge around His people and had watched them for their good, but since they cared more for the things outside the hedge than they did for those within, the time had come for the hedge to go and for the heathen nations to have their day.
Nonetheless, God would see that a remnant from Egypt, those whose hearts were still fixed on Him, would return to Jerusalem (Jeremiah 44:28).
The "rod of an almond tree" is how Jeremiah depicts the enemies that God has allowed to descend upon Jerusalem and His people. They are lions and wolves and leopards who have been watching and waiting and eager for God to remove His protective hand, so they could devour that insignificant little nation, whose only claims to fame are what He had done for them.