here we go again,
athletes take clenbuterol mostly for it's thermogenic effects which result in rapid weight loss. its actions in the body are similar to that of any CNS stimulant. metabolism is sped up and excess calories are burned even while resting or performing activities of daily life. the side effects are what you would expect from a stimulant. jittery hands, headaches, difficulty sleeping, etc. anecdotally some report stomach cramping which wouldn't surprise me either, gastrointestinal distress would seem logical.
clenbuterol can also act to dilate breathing passages and has some anabolic characteristics (muscle building effects). these are hard to quantify but they're generally small and are probably just an added bonus for someone trying to lose weight.
clenbuterol can be effective in a short period of time, a cycle generally lasts at least 2-3 weeks. could it be taken for a few days to drop a kilo or two during a stage race? anything is possible but i doubt it.
again, he didn't take a 50 picogram dose. 50 picograms is what was metabolised (broken down) from a larger dose on the day he was tested. that larger dose was most likely taken weeks or months earlier and transfused back in during the tour.
lastly, the contador case is a strong argument FOR strict liability, not against it.