My history teacher said that's the reason why he always held his hands behind his back, because he was tweaking out. But a different history teacher disagrees.
Leaving the amphetamines aside, he didn't hold his hands behind his back because he was tweaking. Let me just say that you'll find most scholars downplay how much amphetamine he used and their effect, while pop-science and massmedia absolutely tends to portray Hitler as a tweaked out, drug-addled mess.
He had tremots in his left hand (and later leg), which is by most thought to be some type of idiopathic Parkinson's disease, but this is still debated, as every aspect of Hitlers mental and physical health tends to be. By 1942 the tremors were clearly visible.
Holding hands behind your back, or holding onto a briefcase, or a pair of gloves with his left hand would to a degree hide and stop the shaking from being visible to onlookers. Holding his hands behind his back, with his hands grasping each other, was a strategy to stop the tremors, not because he was tweaking.
Here's a clip of his left hand with tremors.
Below is an abstract of a paper published in 2003 by A L Guerrero. He argues that the treatment for Parkinsons Hitler received had as much to do with his symptoms as the disease itself:
Adolf Hitler very probably suffered from Parkinson's disease. The first symptoms of it began to appear in 1937/1938. It is likely that its appearance, and the fear that it caused regarding his survival, lead Hitler to advance his initial projects of military expansion of the great Germany beginning in 1943. Thus, the Second World War broke out in 1939, perhaps quite before the time in which Germany would be prepared.
Chronic treatment carried out with opiates, cocaine, amphetamines, and strychnine may very well be related with a very abnormal judgment of the problems and absence of trust in the advice of his team. With this, he would make military decisions that would end up being ill-fated for his interests and which, after 1942, would lead to a change in the course of the war."
Joachim Fest (author of a very well regarded Hitler biography) cites several sources and says this:
Probably the exact nature of Hitler's illness can no longer be determined, since no examination with a specific investigatory aim was ever undertaken. Because of the extremely inadequate documentation, none of the various diagnoses can be persuasiverly supported or rejected; the principal symptom of both Parkinson's disease and the Parkinson syndrome, namely the shaking arm or leg, can also be caused by many other diseases.
Ian Kershaw is undecided on the issue, he writes that while it is possible Hitler suffered from Parkinsons, it can not be said with absolute certainty that he did.
hi! additional info is welcome; meanwhile check out previous responses to these related questions