First question: http://ruby-doc.org/core-2.1.0/Hash.html#method-c-new
Let’s write a few more sentences. There are two ways to use it

xxx = Hash.new { |hash| Hash.new(false) }

or

xxx = Hash.new { |hash,key | hash[key] = Hash.new(false);nil }

Actually, the hash = Hash.new(false); written in the question is completely unnecessary. The meaning of this sentence is actually
Actually I just want

 Hash.new { Hash.new(false);}

is enough.

Second question: If item_array does not have a member with symbol sindex, then execute the previous @item_array[i] = []. This syntax is to post clauses such as if (or while unless). What it expresses is actually

xxx if condition

=>

if condition
  xxx
end

Essentially, there are a few lines missing. .