The projective buffer transitions presented here bear some resemblance to the easy-first parser by Goldberg and Elhadad (2010), which allows creation of dependency arcs between any pair of contiguous nodes and is based on the idea of "easy" dependency links being created first.
to test this idea, we also conducted experiments with the alternative policy "use buffer transitions whenever possible, regardless of arc length": as expected, the obtained accuracies were (slightly) worse.