Piggyback Warrants
  
See: Piggyback Registration Rights.
It’s like those deals you get sometimes at checkout. Like, "You’ve been such a good customer at our medical-grade deodorant store, here’s a case of foot powder for 25% off." Only, in this case, it has to do with stocks or other securities.
A company issues a warrant to purchase stock. That gives the holder the right to purchase shares at a pre-set price. However, this particular warrant has a sweetener...a piggyback warrant. It's like an extra, added-on warrant.
The piggyback warrant provides a further opportunity to buy additional stock. Again, the stock purchase has a set price. In addition, these warrants often have a time limit.