I will make this lesson short and simple: the doubling cube is a weapon, just like a gun or a knife. It can be used to kill or severely injure your opponent. But you can only use the weapon when you have access to it. And you only have access to the cube if it is in the middle or if it is on your side. Once you give that doubling cube, if your opponent takes it, not only do you no longer have access to the cube, but he still does!
And that, put very simply, is why you should not double too soon. There are many situations where you are leading the game and will win more than 50% of the time. Inexperienced players believe that if they are going to win more than half the time, why not play for twice the stakes? The error in their thinking is twofold: first, backgammon is not about how many games you win or lose, but about how many points you win or lose, and second, if you are in a position where you are likely to win over half the time, part of the reason for those odds is your ability to end the game with the cube at some point. If you give up that ability, you might no longer be a favorite to win at all.
So when should you double? You should double when it hurts! When your opponent has a tough decision whether to take or drop, or when he clearly cannot take and must give you the victory. The "specifics" on when to give and take the doubling cube vary greatly depending on the specifics of the given position... the race, the volatility, the gammon and backgammon risks all have to be taken into account.
Conversely, it is also a mistake to double too late. Let's say, for example, you are in a pure racing situation and you are up many, many pips and if you double, there is no way your opponent would take the cube. Unless you can win gammons or there is some match score reason not to double, you should double and take the win. Of course, if your opponent makes a huge blunder and takes the cube anyway, all the better.
But if you don't give the cube in the above situation, the downside is that your opponent could roll a bunch of doubles and win. So instead of taking your victory, by not doubling you have risked losing.
And there is a third common error relative to giving the cube, and that is giving it when you are "too good". "Too good" means that you are much more likely to win a gammon or backgammon if you play on, and if you give the cube and your opponent drops, you lose the extra points you would have gained. In money play, the general rule of thumb is that you play on without redoubling if you win twice as many gammons as losses. I say it's a "general" rule of thumb because again, volatility and exact checker position and risks must be taken into account. And of course, you should also take into account the possibility that your opponent might make a mistake and take the cube even if you are technically "too good" to double.
In match play, the doubling when too good decision is more complex for two reasons. First, there is no Jacoby rule - you can win a gammon or backgammon even if the cube is in the middle. In money games, the cube must be turned to win a gammon or backgammon. And secondly, in addition to weighing the odds of the given game, you have to consider the score of the match to determine whether to play on or go for gammons. At some scores, gammons are critical, and at other scores, they matter very little or not at all.
What about take decisions? Everything I have stated above about giving the cube applies to the taker in reverse. You might find yourself in a position where you are the underdog, but if your opponent makes the mistake of giving you the cube, now you hold the power…you hold the weapon, and you are the one who can end the match with the cube. You might go from an underdog to a favorite, or near - favorite, simply because you now have the weapon!
So think carefully about the cube, and think of it as a weapon, and use it wisely