Can someone explain to me how the "-" method work on array ?

ArrayEqualTest

require 'minitest/autorun'
require 'minitest/spec'

class ElementTest
  attr_accessor :value

  def ==(other)
    value == other.value
  end

  def hash
    value.hash
  end
end

describe "Test" do
  it "should be equal" do
    et1 = ElementTest.new
    et1.value = "1"
    
    et2 = ElementTest.new
    et2.value = "2"
    et22 = ElementTest.new
    et22.value = "2"
    
    et3 = ElementTest.new
    et3.value = "3"

    arr1 = [et1,et22]
    arr2 = [et2,et3]

    arr2.include?(et2).must_equal true #success
    arr2.include?(et22).must_equal true #success
    arr2.include?(et1).must_equal false #success

    arr1.include?(et2).must_equal true #success
    arr1.include?(et22).must_equal true #success
    arr1.include?(et3).must_equal false #success

    arr2.select{|v|!arr1.include?(v)}.size.must_equal 1 #success
    arr1.select{|v|!arr2.include?(v)}.size.must_equal 1 #success
    
    (arr2-arr1).size.must_equal 1 #fail
    (arr1-arr2).size.must_equal 1 #fail
  end
end