In a nutshell,
so that code preceded by all
jit(1) function call, using his
package from the CRAN archive, will
run faster due to just-in-time compilation of loops and arithmetic
Ra offers to pick the low-hanging fruit for users as loops can be a bottleneck. Of course, as shown in Stephen's case study, using appropriate vectorised expression will often be faster still. That said, for a certain class of problems, Ra should offer a decent speed boost.
Debian users can now just say
sudo apt-get install r-base-core-ra r-cran-jitas the Ra and jit packages in Debian's unstable distribution (and in the case of jit, even in testing).