It is best to start with definitions. An occasion statement introduces the reason for writing. In other words, an occasion statement can be any event, idea, or anything else that gives a writer a reason to write. The statement is usually in the form of a complex sentence and is introduced by one of these words:

If, After, Since, Before, So that, Whenever, As long as, In order that, Even though, Although, Unless, While, When, Even, As if, As, Until, Where, Though, Even if, Because.

Here are some examples:

If you think that it is fine to talk and chew at the same time, then consider what happened to these people. 

Even though wearing a helmet and other protective equipment seem like a hassle, they are meant to protect you in case of an accident.

As you can see from these two sentences, they are setting up an argument, that is, the reason for writing.  

My use of the term "occasion statement" is generally in conjunction with a position statement.  An occasion/position statement is one which addresses a position in terms of a qualifying condition.  The "occasion" portion of the sentence is comparable to the independent clause in a complex sentence and explains the reason for, in this case, writing.  For example:

• If you don't want to be tired in the morning, you should go to bed earlier.
• Whether or not you agree, term limits are not the law.
• Even though wearing a seat belt may be uncomfortable, wearing a seat belt is safer than not wearing one.

Think of the "occasion" portion of the sentence as the qualifying clause for the position which follows.  Hope that helps.