I was asked earlier about putting two index finger pluckings back to back & why not continue alternating the index finger & thumb. It is simple. As was stated above a jig's accent is on beats one & three (or as I count out "1","&", "a","two","&" it is on beats "one" "two")
you simply can't get those beats strong if you just simply alternate the index & thumb. A guitar picker has to make similar changes as he has to pick, DOWN, up, down, DOWN, up, down etc. I hope this clarifys things Charcloth