which is to say 在句子中起什么成分

It's all about dragging and dropping which is, to say, is veryintuitive.

请问老师,如此的句子中which is to say是句子的什么成分? 如何断句来理解 is veryintuitive 这个部分? 

这是说话人的口误。你可以忽视这种句子。