Confusion with Ruby's enum#inject

From the doc about the syntax : inject {| memo, obj | block } → obj

:: If you specify a block, then for each element in enum the block is
passed an accumulator value (memo) and the element

[1, 2, 3, 4].inject() { |result, element| p "executing" ; p "#{result} =>

#{element}" ; result + element }
"executing"
"1 => 2"
"executing"
"3 => 3"
"executing"
"6 => 4"
=> 10

Now while the above is understood very well. below is a bit confusing
me.

inject(sym) → obj

:: If you specify a symbol instead, then each element in the collection
will be passed to the **named method of memo**.

[1,2,4,3].inject(:+)

=> 10

Couldn't understand the line within **. With symbol only, how inject is
working to binary operation + and how it passes the value of enum and
where the operation is taking happening to produce final operation `10`
?

···

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