Parallel apply is an important technique for achieving high speed replication and curing slave lag. It works by spreading updates to slaves over multiple threads that split transactions on each schema into separate processing streams. This in turn spreads I/O activity across many threads, which results in faster overall updates on the slave. In ideal cases throughput on slaves may improve by up to 5 times over single-threaded MySQL native replication.
It is worth noting that the only thing Tungsten parallelizes is applying transactions to slaves. All other operations in each replication service are single-threaded. For a summary of the performance gains see the following article.