The card in the UPS communicates with an agent (software client) installed on your servers. Upon reaching your threshold, the management card will send shutdown signal to the agent software.
The agent software will then instigate an orderly shutdown of the operating system.
You have newer servers, so this should also perform a hardware shutdown via logic built in to the motherboard. Older servers would be left on the screen "OK to shut down" screen, depending on the OS, but would continue to spin drives and fans.