2×4: Image courtesy of Wikipedia .
Edit:
Wikipedia has since corrected itself that this is indeed a 2×6, but you get the idea.
Oh, and the sentence as a whole means that the observation was a big surprise or a great shock; an aha experience or a eureka moment ; an eye-opener .
english.stackexchange.com/users/300/RegDwigнt
A two by four is a common format for a piece of wood : A length of sawn wood of cross section 2 inches by 4 inches, most often employed as structural framing lumber (dimension or dimensional lumber).
"It hit me like a two-by-four" means that you've been hit pretty hard.
In this context, the author uses it as a simile to convey his state of shock after reading the analysis he quoted.
He felt that it explained so well why Asians were often stereotyped as good in science and math that it stunned him.
He explains this further down the page:
I hadn't known about the 1965 Act, and its existence and the above analysis just instantly explained so much so well and without reference to Asians somehow being magically different
In other words, he was really surprised by the quality of the explanation.
english.stackexchange.com/users/3711/Borror0
From Word Reference forums : "Two-by-four" is a standard size ofwood used in the UK building trade.
So it means a piece of wood with sides of two inches and four inches.
english.stackexchange.com/users/127/Mehper C. Palavuzlar
This may have derived from the story about how to handle Mules.
"First" (clobbering mule with 2x4), "you have to get their attention."
english.stackexchange.com/users/597/MickeyfAgain_BeforeExitOfSO
Expression comes from a very old joke: about a farmer with a stubbron mule who would not go forward or backward let alone respond to owners comand.
So the old man picks up a 2x4 and smacks the mule right between the eyes.
Then the mule responded to command.
Onlookers were outraged that the man punished the animal harshly.
The farmer explained that "first he had to get the mules attention".
I suspect over the years people have become more sensitive to this type of 'animal abus jokes' and it has died a just death, the source of this saying , lost.
english.stackexchange.com/users/312172/Marie Terrell
What does this sentence mean?   This observation hit me like a two-by-four   Source .
What does "hit me like a two-by-four" mean?
english.stackexchange.com
<meaning><phrases>
english.stackexchange.com/questions/9373
english.stackexchange.com/users/3854/instantsetsuna
The sentence as a whole means that he was really surprised by the quality of the explanation. A two-by-four is a common format for a piece of wood and "It hit me like a two-by-four" means that you've been hit pretty hard. The sentence means someone is shocked or surprised. The reference to a "two by four" is to a piece of wood to convey being hit by it. Some believe it is derived from a story about a man handling a mule which he punished harshly with a piece of wood and by analogy conveys the sense of shock.
The sentence as a whole means that he was really surprised by the quality of the explanation. A two-by-four is a common format for a piece of wood and "It hit me like a two-by-four" means that you've been hit pretty hard. It hit me like a two-by-four means that the observation was a big surprise or an "eye opener". I can mean that you have been hit very hard.
"It hit me like a two-by-four" means that you've been hit pretty hard.  It means that the observation was a big surprise or a great shock; a eureka moment ; an  eye-opener. It is used as a simile to convey a state of shock. This phrase means that something was a great shock, in this case, an observation.
"First" (clobbering mule with 2x4), "you have to get their attention." Expression comes from a very old joke: about a farmer with a stubbron mule who would not go forward or backward let alone respond to owners comand.
"First" (clobbering mule with 2x4), "you have to get their attention." Expression comes from a very old joke: about a farmer with a stubbron mule who would not go forward or backward let alone respond to owners comand.
It means a big surprise or shock.  The "two by four" reference is to a piece of wood. It may come from a story relating to handling a mule.
"It hit me like a two-by-four" means that the observation was a big surprise or a great shock; an  aha experience  or a  eureka moment ; an  eye-opener . It hit me like a two-by-four means that you've been hit pretty hard.  
This phrase means that something was a great shock, in this case, an observation.