That's not quite true. While Jack labels their product as Tennessee whiskey, they technically follow the rules for bourbon (with the addition of the charcoal filtering). One of the rules for bourbon is that you must age it in a NEW oak barrel. So every batch of bourbon needs to go into a new barrel. If you use a used barrel, it's still whiskey, but it isn't bourbon. That's why there is such a huge market for bourbon barrels, they can't use them again. So they get sent off to other whiskey makers who aren't limited in their barrel choice (scotch is the biggest market), and brewers who want to barrel age. You can get repeatable results with used barrels, and the results can be a lot more friendly as well. michter's us1 american whiskey is an example of a whiskey aged in used barrels that is fantastic. Smoother, sweeter, less harsh, excellent.
To address the person asking why you soak in whiskey not vodka, you're trying to mimic aging the beer in a barrel. Brewers cannot add liquor to their product, so if they want that flavor they have to get creative. When whiskey ages in a barrel the barrel soaks up some of the product in the wood. Then you fill with beer and the wood will trade some of the whiskey for beer, adding that flavor. So you pre-soak the oak pieces in whiskey to mimic this and allow you to add whiskey and oak flavor at the same time during aging. You could use any number of liquors to get the flavor you want. Port soaked oak is also fantastic in a stout.
Don't get hung up on the amount of whiskey to soak the oak in. Basic rule is just cover them in whatever container you have and let it sit for a while. Add the cubes/spirals/chips whatever you used, and then after it's aged on the oak a while taste and decide if you need to add more of the liquor to get the flavor you want.