I have heard stories that certain street drugs make people incredibly strong as well as resistant to pain and I am curious what the physiological basis is for this.

I have heard a few stories from people that work in corrections, as well as others, saying that PCP makes people strong to the point where they use muscles to the point of severe damage without any perception of pain.

Do these sorts of things block or affect neurotransmitters in such a way that the pain is not perceived or is it another mechanism that allows it to happen?

Yes. It pretty much "unlocks your body". It causes massive amounts of adrenaline to flow through your body, which causes abnormal strength and ignorance to pain. This is another reason why PCP victims die from cardiac arrest, the heart can't take kind of output.

I think it's because it removes one key element: fear. PCP just causes a massive rush, and ability to ignore pain and fear.

It probably has a big effect on how efficient your CNS recruits motor units.

Because normally not all mu's are recruited during a lift or movement or whatever, but i reckon during these momental times, for whatever reason the nervous system becomes extremely efficient at the mu recruitment, therefore you get some 'super human' strength etc.

Like you hear about people lifting cars etc to save someone.

I think that could be a factor, could be wrong