Invocation resembles the use of language itself. When using language, there are often many ways to get a message across and deliver an intended meaning, and some of those ways are more effective in certain situations and with certain people, and other ways work best in other circumstances. It is also a matter of personal style and choice that determines how and what is said, to whom, etc.
Regardless of the particular language that is used, the end result is communication. Language choice is contingent upon the words chosen only as long as the words convey the meaning intended.
You are driving to an unfamiliar part of town with a friend, who knows the directions and is guiding you to your destination. Suddenly she says, "Okay, go left!"
You are startled and not sure what to do. You see a street 100 feet ahead, a driveway 20 feet ahead, and a small dirt road leading into a field between the two. Based on her intonation and excitement, you assume she meant the closest option. You start to slow down and head for the driveway, but she directs you to turn on the next street instead.
The confusion is based in a lack of specificity. Here are some other ways she could have communicated the same meaning:
"Turn left at the next street up ahead."
"Take the left after we pass by this field."
"In about 100 feet, turn left."
"Take your next left on Maple St."
"After passing by a red barn in a field on your left, you will see a row of large Maple trees; turn left at the end of the row."
"Once you pass the dirt road on the left, make your next left."
In all of these examples, what she says is important insofar as the meaning of what she says enables you to understand her. Simply, communication and guidance.
Your friend giving you directions is like the shaman, and you are the spirit that she is guiding. She knows where she wants to go and how to get there, but she needs your help. Her directions are invocations. The words and phrases found within each invocation are spirits - guiding spirits that provide specific tasks to accomplish. The spirits in each invocation are different - some use Maple trees, street names, distance or scale, landmarks and symbols - but they all suffice to get the job the done.
This is what invocations are, and what they do. They keep things moving. They bring one story to its completion and help facilitate the next. They help you move out of a space filled with doubt and into one of clarity. They help you get what you need because you know how to ask for it.
Invocation resembles language because language is invocation. The way you use invocation in shamanism and with spirit guides is a process of learning and communicating in a new language. When you communicate with guides, clarity and specificity in meaning will ensure that your guides understand you and perform as the invocation intends.
Invocations guide the spirits, so that the spirits can guide you...