The word sounds like cutting, yet hand trimming involves no scissors. The hair is pulled out, with the fingers or a small stripping knife. Most people wince the first time they hear this. Understandable, but unnecessary.
To see why, look at how a wiry coat works. The hard hair of many terriers grows, reaches its full length and then dies. So it's already dead when it's trimmed, and only sits loosely in the skin. Done properly, it doesn't hurt. Some dogs even doze off, especially if they got to know it as puppies.
Why not just cut it? Because the result is different. Scissors shorten the hair, but the dead hair stays in the skin. Trimming makes room for new hair and follows the rhythm the coat already has.
That doesn't make cutting bad. Plenty of dogs have hair that keeps growing and would soon mat on the legs and belly without scissors, or see nothing through their fringe. For them, a cut is exactly right. Both have their place, just not on the same coat.
It gets interesting with mixed breeds, who often have a bit of everything. No breed list helps here. You have to feel the coat, and that usually tells you more than any pedigree.