See README for basic information on distcc.

Distcc's "pump" mode improves on plain distcc by distributing not only
compilation but also preprocessing to distcc servers.

The pump mode uses an "include server" process that runs during
the build. The include server parses and analyzes source (including
header) files. It runs on the workstation that initiates the build.
The include server analyzes each header file only a few times during a build,
sometimes just once. In contrast, during ordinary distcc operation,
the preprocessor examines each header file multiple times, even hundreds of 
times for a large build.

In pump mode, a static analysis algorithm inspects each #include directive
and computes a superset of the possible values of its argument.
The resulting dependency graph among header files persists during
the lifetime of the include server, which then acts as a cache for include
analysis.

The include server compresses source files into a temporary directory
as they are encountered. In this way, a given source file is
compressed only once during the build.

It may happen that a header file is included via an absolutely
specified include directory such as -I/absolute/path. But on the
compilation server the path -I/absolute/path does not exist; instead
the server places foo.h under /server_temporary_path/absolute/path for
some /server_temporary_path root directory. This directory has no
meaning on the workstation. Before compressing foo.h, the include
server therefore inserts a #line directive in foo.h, to inform the
preprocessor that the real location is /absolute/path.

The distcc client asks the include server for the list of
compressed files that constitute the transitive closure of the source
file to be compiled. It then spools these files to a distcc
server. The distcc server unpacks these files in the
/server_temporary_path directory before preprocessing and compiling.
The server also rewrites include options, such as -I's, to reflect the
new locations of the files on the server. The .d and the .o files are
both rewritten as necessary to refer to client-side filenames and 
returned to the pump-mode client.

Pump mode is able to distribute compilations up to 10X faster than
plain distcc. But because building also involves linking and perhaps
generation of source files, the overall speed-up of the build time is
variable.

The pump mode was developed to be used with large clusters of distcc
servers, providing hundreds of CPUs. With versions of gcc >= 4.1.1,
pump mode will probably not show major performance gains using
clusters of less than ten CPUs. The preprocessor running on the
workstation is fast enough to keep that many machines busy.