So how then does he control the future? Well we don't exactly see the future of Animal Farm past what we see in this novel.
George Orwell wrote this work in 1943. Stalin, the allegorical character Napoleon, remained in power until 1953. During that time after WWII and after the book Animal Farm was written, Stalin rewrote and edited the book entitled Falsifiers of History which revealed Nazi-Soviet relations and pointed the finger to the west for the second world war. He also put in place the Doctor's plot which was a plan to kill lots of Soviet officials. The actions after Animal Farm that took place were almost more severe than those it symbolized. The future was definitely in Stalin's hands and his power increased until his death.
Napoleon represents Orwell's quote in the fact that the consolidation of power seems to indicate a rewriting of past in order to secure a future of power. When Napoleon assumes power, he is driven to ensure that his reign is uninterrupted. This requires him to convince his subjects that his tenure as leader is one that has been a historical development, justifying its continuation into the future. Along with Squealer, Napoleon is able to reconstruct the past to substantiate his actions. In the process, this allows him to eliminate all competition and continue his reign into the future. When political rule must seek to substantiate itself, it does so, according to what Orwell has presented, by ensuring that the past is airbrushed, in a matter of speaking, to highlight that the current rule is continued into the future. It seeks to rewrite the past in accordance to the present and the future.
In the book, Napoleon does a lot of lying about the past. Because he lies about the past, you can say he is controlling it (because the animals only know his version of it -- they forget what really happened).
Think about some of the ways he changes the past. He makes everyone believe that Snowball was an agent of the enemy even though Snowball was really trying to help the animals. He keeps changing the commandments until they come to say something very different from what they used to say.
So he is controlling the past, making it look like things happened some other way than how they really did.
How does this help him? By making Snowball look like the bad guy, Napoleon makes himself look better and makes the animals be loyal to him. By changing the commandments he makes it look like Old Major would have approved of what he is doing. In both cases, that makes his power more secure.