With this free product, you can transfer between GVB night busses free of charge, when paying your bus ride with e-purse balance (saldo).
If you want to transfer at night between GVB night buses, then put the free 'Night bus balance' product on your OV-chipkaart. This makes your transfer free of charge. Without this product on your card you pay € 4,32 for a transfer.
The product does not cost you anything. You load it once on your card.