I just assumed it was wires, but I was never sure. It's what PositivelyEmerald said: when would he have had time to get harnessed in to said wires? Javert comes on for the last little confrontation with Valjean and stays onstage til the end of the Suicide, and he doesn't stand in one spot for the duration of the Soliloquy.
Stuff like this is why I love this site (long-time lurker, just now getting around to posting), just the fact we're all analyzing this one effect.