class Sass::Tree::Visitors::Perform

def visit_if(node)

otherwise, tries the else nodes.
Runs the child nodes if the conditional expression is true;

def visit_if(node)
  if node.expr.nil? || node.expr.perform(@environment).to_bool
    with_environment Sass::SemiGlobalEnvironment.new(@environment) do
      node.children.map {|c| visit(c)}
    end.flatten
  elsif node.else
    visit(node.else)
  else
    []
  end
end