Hi, please excuse what may be a daft question, but

Hi, please excuse what may be a daft question, but what does the piggyback mean in this context?

I've have searched and found at least two different interpretations of what a stock piggyback and piggyback rights is/are and would like clarification on its meaning in the respect of the post by Rosner.

Thanks in advance for any helpful, constructive replies.

Chimpus.