Piggyback registration is the right that lets preferred stockholders include their shares in any registration statement the company files. It applies to the company's own offerings (IPO, follow-on, secondary) or to other holders' demand registrations, is generally unlimited in number, and is subject to underwriter cutbacks that can reduce or eliminate piggyback allocations in oversubscribed offerings. It is the more flexible and frequently exercised of the registration rights, particularly valuable to investors who want to participate in selling alongside the company without forcing their own demand registration.
The mechanic of a piggyback registration: