If you have been injured while on Active Duty, have recieved a disability rating from teh VA and you are a reservist then you can request form 21-8951-2 from the VA.
This form will allow for you to recieve a pro-rated disability compensation. The way the VA looks at this is that you are only active duty for two days a month so they take your current compensation payment divide it by thirty and then multiply it by twenty-eight.
Your unit MUST submit VA form 21-8951-2 every month or you run the risk of creating an over payment.
You would submit the paperwork through your Unit Administrator.