In explaining the tarrar, the textbook describes someone who secretly cuts a pocket or garment to remove property. The method differs from the ordinary picture of theft, but the covert taking of another's property is particularly evident.
The source's commentary points to an additional resemblance to the defining idea of theft in this case, which becomes relevant when assessing inclusion under the term.
The legal-language lesson is that knowing the usual meaning is insufficient for judging every unusual instance. The relevant features must be established before deciding whether the category applies.