BSV?
Don't know what BSV is, but here is my take on double downs.
Count systems and basic strategy call for you to double down when you have an advantage on the hand. If you didn't have the advantage why would you double down? (Yeah, I will probably lose this hand, I would like to bet more money please.) So after seeing your two cards and the dealer up card, you determine that more times than not, you are going to win the hand, so you increase your bet.
Casinos realizing you are in an advantage situation and are increasing your bet, create a restriction. You can only take 1 card. So if you double down on 9 and get a 2, you cannot hit again, you have to stand. This is the trade off. You give up the advantage of maybe hitting a couple of times and being more likely to win the hand, by just taking one card for more money bet.
Now casinos being really nice, will let you take the full disadvantage of only getting 1 card, but allow you to only take a small portion of the advantage, by allowing you to double for less.
What nice guys. (Yes, I have the advantage, but I only want to win half the money I could, and maybe the next hand I lose, you will give me back half, right?)
Bottom line. If you are in a double down situation, always double down for the full amount. Hell, double down for 10x your bet if they will let you. If a double for the full amount is overbetting your bankroll, then you bet too much on the hand to begin with.
If you don't take full advantage of the rare times you have a slight advantage, the casino will be glad to take your last dollar on the many times they have you at a slight disadvantage.