The easiest example of a trigger point is a muscle knot. Trigger points frequently occur at specific places that were determined by Dr. Janet Travell in the early to mid-1900s. These trigger points cause very specific pain patterns that are sometimes not located in the vicinity of the trigger point. A common example is a temple headache that originates from the neck portion of the trapezius muscles.
The metaphor I use is that of a dam in a river. When a dam is installed, not many changes are seen at the dam, but rather upstream where the water no longer flows like it once did. The needles work by releasing the trigger point and soothing the pain. Many times, the muscle will quiver as it releases and the pain relief can be instantaneous. There can be some soreness afterwards but it feels like the same soreness one feels after going to the gym. The muscle has just released tied-up toxins and needs time to move them.