Just flashes a red light when I power up the C64. Has a small button on the top. Hadn't realised just what it did.... says NIC VALLEY PRODUCT.
It doesn't look like the official ESM add on for the expert, but it could be a clone or something. It's use shouldn't just be limited to expert cartridges either.
Freezer cartridges that rely on the restore button can be blocked by getting the CIA to generate an NMI but never acknowledge it.
http://codebase64.org/doku.php?id=base:nmi_lock (it's also used to stop demo timing being affected by people randomly hitting restore). What the ESM cartridge does is forcibly drag the NMI line high again so that the CPU will detect the interrupt (IIRC it supplies more current than the CIA can sink to ground). Trilogic produced an addon and included it in later expert cartridges.
The light will flash every time there is an NMI. So if you load up something from the 80's with samples then it will flash every time a sample is played (savage/stormload/turbo outrun intro etc).
