The printer window can automatically pop up when the emulator prints something. Use Printer → Auto Popup to enable this feature. You can also simulate a powered-down (or disconnected) printer with Printer → On. The printer window is a simple ASCII view of the printer output. It works great for BASIC listings and such but for more complex output see below for how to fully emulate a printer.
As a convenience the emulator goes into turbo mode whenever output is sent to the printer. This "printer auto-turbo" can be turned off and on with the -pt and -ptx command line options or via the Printer → Auto Turbo menu entry.
Like the serial ports the printer port can be mapped to a TCP/IP connection. To listen for a connection use -p @port or the "TCP/IP Listen" option in the Printer → Connection... dialog. To make a connection use -p host:port or "TCP/IP Connect" in the connection dialog.
To attach an emulated MX-70 printer use -p :mx70 on the command line or use Printer → Connection... and select it from the drop-down.
The Electric Crayon colour display actually attaches via the printer port. The idea was to make it easy to program. For example, a BASIC LPRINT"ERS" will erase the screen. See the section on the Electric Crayon for some details on how to use it.
In batch mode any TRS-80 printer output is written to a file called trs80-printer.txt.