This will cause the [https://wiki.openwrt.org/doc/techref/bootloader/uboot U-Boot] bootloader to drop into a command line.
==== TFTP Serial Output ====
When booting with the reset button held down on a Ubiquity NSM5, the following output is displayed.
U-Boot 1.1.4-s958 (Jun 10 2015 - 10:56:20)
DRAM: 64 MB
Flash: 8 MB (0xc2, 0x20, 0x17)
Board: Ubiquiti Networks AR9342 board (e855-22585.1122.0030)
Hit any key to stop autoboot: 0
Setting default IP 192.168.1.20
Starting TFTP server...
Using eth0 (192.168.1.20), address: 0x81000000
Will reset device configuration (Reset button active after 10 seconds).
Erasing sector 123..126
First 0x7b last 0x7e sector size 0x10000
==== Transfering file to U-Boot over Serial ====
U-Boot supports two protocols for transfering files: kermit or y-modem. Minicom does not support the kermit protocol, however it does support the y-modem protocol.