Resource Reservation is the feature of Circuit Switching. It is applied in Physical Layer.
In Circuit Switching, when the first packet of the stream goes through the router , it informs all of them that more packets are following it. The memory and CPU is reserved for the following packet for processing and forwarding.It is called circuit setup. When the circuit setup is done, all the packet follows the first packet. (Next packet follows previous packet)
That is why,
1) Packet does not need header except the first packet in circuit switching
2) Packet is recieved in-order at the reciever side.
This the reason, while large amounts of data need to be sent,the packet switching is used.