File transfer using YMODEM sz

I'm trying to upload a firmware file over a serial connection to a device that requires YMODEM protocol, from a Raspberry Pi. After a lot of digging, I keep finding that the sz --ymodem [file] command is the tool to do this. I've already managed to just communicate with the device using this example, but I'm having no luck with sz. I've read through the sz documentation and it leaves me with a question. How do I determine if it is sending to the device? It is plugged in via USB and has port /dev/ttyACM0. Other examples talk about sending from a remote host to a local host via sz by default, but that's as deep as any explanation goes. The device has a command which tells it to anticipate a file transfer; I believe this takes the place of rz, but the device documentation says it "Prepares the device for YMODEM transfer via HyperTerminal." I've sent it the files via HyperTerminal and a proprietary program successfully, but I need to be able to do it on the Linux command line. I'm sure this is a case of inexperience and I'm missing something obvious, but how can I fully execute this file transfer from start to finish / what am I doing wrong?