Apparently, digging thirty-foot tunnels is tough work, especially when you don't want to use a shovel. He concedes to practicality, as long as they can pretend they dug him out with a knife and not a pickaxe.
Huck takes this as a sign that Tom is just full of principles.
The boys include Jim in plotting their various machinations, many of which include hiding things like brass candlesticks in his food.
When Nat (the superstitious slave) starts getting suspicious, the boys blame all the mysterious happenings on witches.
Not to worry. They'll bake a witch's pie to placate the spirits.