The mechanism for specific preworkout supplements will vary based on the supplement in question. It helps to frame the question around certain preworkout supplements specifically. I can go down a common list:
- BCAAs can help with protein synthesis and uptake for people with low protein in their diet (below 1g/kg body weight). In novices it can help lower the intensity of DOMS. However, for people who consume over 1-1.5g/kg protein they already get enough BCAAs in their normal diet and supplementation does nothing more than provide a placebo effect.
- Caffeine is a nootropic stimulant with well studied affects for increasing physical strength and endurance. Continued use causes you to be tolerant to the substance.
- Creatine is a well studied supplement that increases your pool of creatine phosphates, allowing you to regain ATP more quickly. It increases power output, available muscle creatine content, and water retention.
- Taurine is a semi-essential amino acid that assists in anti-oxidant defense systems. It's primary benefit is increasing blood flow.
There are other pre-workout supplements and supplement stacks, but research is still being compiled. For example Nitric Oxide (NO) has a lot of marketing behind it, and is supposed to improve blood flow--however independent research is still being compiled.
One component not yet covered that is present in many pre-workout stacks is dextrose or glucose. This is simply a form of sugar and works by providing a direct source of quickly metabolized energy.
NOTE: do not use any pre-workout that contains DMAA as this is considered harmful by the FDA, and has been implicated in psychiatric disorders, heart problems, nervous system disorders, and death.
If you are unsure of how any supplement works, check out Examine.com. They compile independent research and grade how useful the research is and the amount of effect the supplement had. There are a number of supplements that simply don't do much.
Lastly, it is not uncommon for the placebo affect to affect many friend's recommendations. They hear it's supposed to do something, so they make it true. One such example is a pair of studies where all the participants were told that there were going to be two groups: those taking steroids and those not taking steroids. All the participants were administered a placebo and told they were in the steroid group. All the participants saw increases in ability in line with steroid use. Then one group was informed that they really weren't in the steroid group and they were getting a placebo all along. That group dropped back to their pre-study capabilities. This was in face of the fact that the participants saw real improvements without steroids. The placebo affect can be very strong.