The only mode the Proc backend currently knows about is
compile.
The Proc backend can be configured to not start syntax check if it
thinks the compilation is in progress, by testing the
compilation-in-progress variable. The reason why this might be
useful is saving CPU time in case both syntax check and compilation
are very CPU intensive. The original reason for adding this feature,
though, was working around a locking problem with MS Visual C++
compiler. The variable in question is
flymake-proc-compilation-prevents-syntax-check.
The Proc backend also provides an alternative command for starting
compilation, flymake-proc-compile. It just kills all the active
syntax check processes before calling compile.