Hot oil penetrates the wood and seals it more effectively. When you put the piece in the oil you see lots of little bubbles coming out as the air and water in the wood is replaced by the oil. Some bits more or less fizz! I leave it in until the bubbles stop. It gives a very good oil seal. It does darken the wood more than a cold oil finish though, which is not always desirable.
I do it by putting a whole bottle (or more) of walnut oil in a solid pyrex microwavable jug, for 3-4 minutes, until the oil is hot but not boiling. I then pour the oil into a tall glass with the spoon in it, as the narrower shape of the glass gives a greater oil depth.
For things like bowls I tend not to use the hot oil treatment, but if you want to you can sit the bowl in a metal tray and fill it with hot oil, and then pour hot oil over the outside afterwards.
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