Doesn't actually do what you think.
Modern UNIX doesn't really support the classical "ram disk", and you wouldn't want to use one in most circumstances: recently accessed files are going to be in RAM anyway, because UNIX uses all free memory as a cache for disk files.
Instead, on UNIX, what you do is create a temporary file system in virtual memory. It gets paged in and out to your swapfiles like anything else created in RAM, and its only performance advantage is that the file system it's using (tmpfs) doesn't expect to survive a disk boot, it doesn't bother to make the extra disk writes that a regular file system uses to ensure that it's consistent and intact after a boot.
This utility doesn't do that. What it does is create an un-named ".dmg" image... a virtual file system in memory, like the ones disk burner uses. Then it creates a regular file system inside it. It uses a script taken from the Darwin man page for "hdid", slightly modified to allow you to specify a different size and name.
I can't imagine this actually being faster than a real file system on disk over the long term. Right after it's created, if you have a LOT of memory, it'll seem pretty fast, but as it gets paged to disk it'll get slower and slower. It's got all the overhead of a real file system, and it forces pages out of your disk cache, and all the clever algorithms the file system uses to improve performance will just lead to extra copies between swapfile and memory. It's useful as a way to mess around with disk images and to test software, I suppose, but it's not going to act like a classical RAM disk that sets aside a dedicated chunk of RAM and runs at RAM speed all the time.