What does this sentence mean?
This observation hit me like a two-by-four
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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.
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.
From Word Reference forums:
"Two-by-four" is a standard size of wood used in the UK building trade.
So it means a piece of wood with sides of two inches and four inches.
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.