Damn me! I misunderstood your post thinking you were asking about a varistor
. My eyes must be in need of glasses, my brain may have some permanent damage too.
Pickit's comment made me realize that you were asking about something that resembles a variac
Sorry about that, please ignore my comment below which is of no relevance to a variac but a varistor.
Basically variacs are used to supress unwanted voltage peaks by being inserted parallel with the input voltage. Using a 120VAC rated variac with 220VAC means it will always be exposed to peak voltages (very high compared to its rating) and get burnt eventually.