When you enable Dynamic Branch to Branch VPN, the first packet goes through the Cloud Gateway (or the Hub). If the initiating Edge determines that traffic can be routed through a secure overlay multi-path tunnel, and if Dynamic Branch to Branch VPN is enabled, then a direct tunnel is created between the branches.
Once the tunnel is established, traffic begins to flow over the secure overlay multi-path tunnel between the branches. After 180 seconds of traffic silence (forward or reverse from either side of the branches), the initiating edge tears down the tunnel.