Autumn Equinox are a three-piece of hurdy-gurdy (the "wheel fiddle"), nyckelharpa (the Swedish "keyed fiddle"), and upright bass. We play tunes from and inspired by Celtic and Eastern European traditions, and the hypnotic drone of the hurdy-gurdy is a common thread. We often play tunes by other hurdy-gurdy composers like Andrey Vinogradov and Nigel Eaton as well.
It appears to be a sort of polyphonic hurdy-gurdy: one tuned string per key, and depressing the key engages that string with the wheel-bow. The wheel appears powered by electricity, and the foot pedals appear to either control the pressure or speed of the wheel-bow.
In contrast, the hurdy-gurdy's (and nyckelharpa's) keys shorten its melody string to increase the pitch, like fretting a violin or guitar. The wheel is manually cranked, and the rhythm of the cranking is used to adjust tone and engage the buzzing of the trompette. The HG is monophonic in that you can't dynamically play chords, though you can engage multiple melody strings simultaneously (usually octaves or fifths).