It's All About the Information
It is a general rule that you can't add further material to a patent application once the application is filed. This is particularly the case with international applications.
So, if you are thinking of instructing your patent attorney to file an international application, I strongly suggest that you review the provisional patent application to make sure that it covers all the technical information. Make sure that you do that as early as possible so that your patent attorney will have enough time to prepare the documentation. You should also be thinking about alternative examples that might be used by competitors to compete against you. If possible, include sketches and as much technical information as possible concerning these alternative examples. Usually, you can only get patent protection for material that has been "enabled" in the patent documentation. This has to do with the basic premise of patent law, which sets out that your monopoly is a reward for the technical contribution that you make to a particular field.
You should be proactive. Time and time again I remind clients to read the provisional specification to make sure that it covers all the information that they consider relevant. Any information that is not included in the provisional specification on which the international application is to be based must be novel on the date of filing the international application if it is to form the basis of your protection. Remember that you paid for the preparation of that documentation and you need to take ownership and have a full understanding of that documentation. This means that you need to understand the definition of the invention, set out in the claims, even if it does seem complicated. Remember that your protection is defined by that material.
I like to think of the information in a patent specification as a ball of Play-Doh. You can do much with Play-Doh, but you can't add to your ball it once the patent application has been filed.