They are able to do this by creating a pressure differential between the two bottles, caused by simply cooling the receiving bottle.
15947 Nitrous Express Bottle Insulated Jacket for 2.5 lb Nitrous Bottle
15945 Nitrous Express Bottle Insulated Jacket for 10 lb Nitrous Bottle
15946 Nitrous Express Bottle Insulated Jacket for 15 lb Nitrous Bottle
Nitrous Express Bottle Insulated Insulator Jacket for 10LB Bottle NX-15945
Nitrous Express Bottle Insulated Insulator Jacket for 15LB Bottle NX-15946
20421-00 Nitrous Express Proton Plus Nitrous System - No Nitrous Bottle