You have probably only seen this annotation in one place: the last section of Chance's Variations on a Korean Folk Song. Wikipedia tells me that con islancio means "with impetuousness", and my usual online musical dictionary agrees.
Well, at the rehearsals of the Great American Community Band, we heard something different. Yes, Variations was part of our concert lineup, and when Colonel Gabriel came to the con islancio section, first he made a jibe at younger conductors who tell their bands that this marking means "like the islands", but then he explained to us the meaning of this annotation in a much different way. I can't quote him directly, but I can give you the gist of his explanation.
"islancio" is like a lancer, or someone throwing a lance. It builds up -- starts with slow muscle movements in just the fingers, then the arms, then the shoulders, the torso, the legs; eventually the whole body is moving until the lance is releasedIf you listen to the very last variation (3:10 in the video below), the instruments add in one by one until, finally, the bass trombones and lowest brass drop in on a Bb -- this is the release of the lance.
And now, a recording with a figure skater, because if you're like me you never conceived of this piece as a skating routine, but now you will never be able to think otherwise.