Where Do I get it?
The ROM images (and associated sources & Control Panel) can be found here
What Does It Do?
Generically speaking, the ROMDisk is a modified IIsi ROM that includes a driver to boot from a disk image residing in the extra space of a ROM SIMM. It allows booting the disk image (typically 1.5MB or 7.5MB depending on the size of ROM SIMM) directly from ROM, or copying the disk image from ROM into RAM so the boot device is writeable (this is required for AppleShare among other things).
The modified ROM image also disables the checksum validation and memory checks for the machine. If you've got a large amount of RAM installed in your machine, you'll notice dramatically shorter boot times due to disabling the RAM check.
How Do I Create a ROMDisk Image?
Please see the Custom ROMDisk Setup page.
How do I boot from the ROMDisk?
During boot, ideally after the gray screen has been displayed and before the Happy Mac/Flashing Question mark icon has been displayed, press and hold the following keys:
'r' will boot from the ROM disk image from ROM. You will have a locked volume. This uses no additional RAM, but you won't be able to use things like AppleShare that require a writeable boot volume.
'r' and 'a' will boot from the ROM disk image, and between the "Welcome to Macintosh" screen and the desktop being displayed, the contents of the ROM disk image will be copied into RAM. You'll notice a brief delay in the boot process while the copy occurs. You'll get a writeable boot volume, and will be able to use things like AppleShare (if it is installed). The contents of the RAM disk will be lost on reboot.
Are there any compatibility problems with accelerators?
It is known that the Daystar Turbo 040 maps its ROM into the $40880000 to $408A0000 address range, which will conflict with the beginning of the ROM disk image. Other than that, we don't know. The Daystar accelerators, and possibly others, are tied to specific ROMs and compare the checksum field against a table in their own ROMs. Since the ROMDisk modified ROM has the checksum verification disabled, the checksum field in the ROM remains that of the stock IIsi ROM, so the accelerator should recognize the ROM. However, interactions beyond that are untested at this point. None of the developers have accelerators.