This could be a very formal way of asking you to follow an instruction such as "Please stand up, if you will?". When used this way it is usually expected that you will comply with the request and replying with a "No" could be considered rude. Usually if a request is made such as this, The person making the request is authoritative and the request would be something that you would be expected to complete. As always, if the request is unreasonable then saying "No" and not complying would be acceptable and would not be considered rude as the person making an unreasonable request would be the rude person. This usage is not common but is certainly used. More commonly when somebody finishes a sentence with "if you will?" they would be referring to how your actions will affect their own decision. Maybe your friend would like to try a new food but does not want to do it alone, he could say "I'll try it if you will?". In this context answering either "yes" or "no" would be suitable, you could be like "No, that does not look good" and neither of you will try the new food, or you could say "Yes, I will try it" in which case both of you will now try the new food. This can be common among friends when making plans to go out, for example if you are talking with a friend about a concert performing in town, you may want to go to the concert but don't want to be alone, so you would say "I'll go to the concert if you will" which would be the same as "I will go to the concert if you are going also". In this context it's not a direct question since you are not actually asking your friend if they are going, you are simply stating that you will go if they will, however, it is common that they will answer as if they were asked if they are going. Another usage could be confirming something that you ask of someone else however I've rarely seen it used this way. In this way "if you will" could mean "yes I will comply with your request if this is your will" or "if this is what you want".

It is usually used to make a request of someone without making it an actual question. I don't think it's a common construction though.