It’s difficult to nail down why certain free-jazz sessions work and some don’t. Musicians listening to each other and responding inventively is key, but there’s still a lot of mystery in the process. One thing that’s clear is that saxophonist Paul Flaherty and drummer Randall Colbourne, two New England-based out-jazz veterans, excel at the give-and-take that happens in this open-ended sound arena. 2008’s Bridge Out! is their first release in several years, but the pair have worked together many times and their musical empathy is striking. A couple of tendencies that can plague these kind of exchanges are aimless drift and playing at only one level of intensity, but these guys sound both focused and flexible. Flaherty can let loose a flurry of agitated cries, but he also knows when to hang out on one sustained note. Similarly, Colbourne may rivet the listener with a few well-placed strikes or unleash a torrent of energetic drumming that delights with its flow, rhythmic shifts, and timbral variances.