How does a router monitor traffic between two devices on a switch

I recently set up a small business network, and am constantly leaing more about how networking hardware works. It is my understanding that a Layer 2 device (Dell 48 port managed switch in this case) will switch traffic based on Mac Address once it leas which devices live down each port. This way not every device in the network sees every packet (obviously broadcast and multicast can be exceptions to this). In this case, I have a Dell SonicWall router connected to the switch serving DHCP. On the SonicWall there is a packet monitor feature that lets you watch for packets originating from or going to a specific IP. If I ping one computer from another computer, both connected directly to the switch, I assumed the router shouldn't be able to see the traffic. However, the packet monitor can see each ping as it happens. How is this possible? All of the traffic in the switch can't be piped back down to the router. The switch as a 100+ gbps backplane and the router is connected over a single gigabit port. Networks would never work if every packet had to be forwarded to the router first.

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