It's like a low D whistle with three normally-spaced holes for the left hand and two for the right, with a gap where the F hole would be. They are neither semitonally spaced nor arranged for a pentatonic scale. You play it a bit like a three-hole pipe but with extra holes, so for the top end you use both hands to persuade the higher harmonics to sound, like the more exotic high register stuff on recorders and in classical flute playing. Simple open fingering doesn't cut it. I can get the lower octave with no trouble, but the folk repertoire goes a lot higher.

Picture and sound sample (from a different maker) at http://www.aural.hu/kaval%20gyumolcsfa.htm