The affirmation, within the context of the text, is as follows:

One of the strongest affirmations which you can use for the purpose of strengthening the will and realizing your power to accomplish, is, "I can be what I will to be."

Every time you repeat it realize who and what this "I" is; try to come into a thorough understanding of the true nature of the "I"; if you do, you will become invincible; that is, provided that your objects and purposes are constructive and are therefore in harmony with the creative principle of the Universe.

To me, it's not about exercising your force of will, in the sense that you are going to take action to achieve your result. Rather, it is about changing your perspective, about making a decision to change your way of thinking.

So the affirmation could also be read as: "I can be what I decide to be."

But read the entire chapter. And try to think about the affirmation within the context of the whole chapter. You're not going to get the insight you want by looking at the quote in isolation.

Part 4 section 9 :Master Key System

One of the strongest affirmations which you can use for the purpose of strengthening the will and realizing your power to accomplish, is, "I can be what I will to be."

I think, here Charles F. Haanel is asking you to identify with the things that CAN happen in your life with the same sureness and confidence as you would identify with the things that WILL happen in your life.

If you are working for a living you WILL be at work tomorrow morning. You don't go to bed the night before wondering and worrying if you CAN be at work tomorrow morning. There is no room for second guessing here for the commitment to your employer has already been made. Come hell or highwater, you WILL be there at 9.00am.

This is the sureness and conficence with which you are supposed to look at the things that you know you CAN do (but probably haven't done yet)

I think the purpose of the affirmation has more to do with the 'I' part of it than the 'can' or 'will' part. Every time you think/recite the affirmation you can/should be realizing/thinking about who and what this 'I' is. If you do, then you WILL become invincible.

Only you can answer that question, and the only way to answer it is to follow the simple instruction given. You will have no real practical realization of the statement until you do, no matter how explicitly it is explained to you. But remember also that you take out according to what you put in. Make the effort now.

The will refers to concentrated thought. It simple means that if you have a desire you must maintain the thought of that desire until it manifests. The affirmation that is quoted in the book is intended to help you maintain the focus.

You can start by thinking of what you want to become professionally, or what you want to manifest in your life, and you can start your plan of action with a step by step focused block as to how you proposed to succeed at your goals.

You can also start a running list of the things that you want to manifest, and then you can go down the list, and do the process of elimination of the least important to the most important etc, but you must a plan in place to work from.

And remember a plan always starts with that first idea to work from. So once you get organized, and you put your priority in order, you are on your way to manifesting your desire.

In general affirmation is a tool to help you to concentrate, and to focus on your intention. Hope this helps.