What are plasma extenders 
Without going into toomuch details, they are substances that atract water where it goes. Plasma expanders used by some bodybuilders end up in the bloodstream and muscle, so they pull water out of the subcutaneous (under the skin) compartment and into the bloodstream and/or muscle. This has two "cosmetic" effects: 1) it removes water from under the skin, giving a dryer look 2) it increases blood volume so vascularity is increased.
It works great *IN THEORY* but in reality proper timing is extremely difficult to achieve, not to mention that there are some risks involved, especially with bodybuilders using a lot of anabolic steroids which increase blood thickness (by increasing hematocrit level, much like EPO does) . Super thick blood increases the risk of heart problems (since the heart has to pump extra hard to get the blood moving). Several guys (your hero Arnold S. was actually one of the first to have this done) have/had blood removed periodically to avoid complications due to thick blood. A lot of modern bodybuilders "give blood" every 2-3 months to lower their hematocrit level (the hematocrit level refers to the percentage of red blood cells vs. plasma blood volume... the more red blood cells you have, the thicker is your blood).
Cyclists are tested for hematocrit levels before most big races... first to try to detect possible blood doping and/or EPO use, but also because an excessively high hematocrit level (over 45-50%) is very dangerous and could lead to heart problems during the race, especially as the athlete et dehydrated (reducing blood plama level, thus further thickening the blood). A cyclist who is found to have high hematocrit levels before a race, even if there is no trace of EPO doping cannot participate in the race. That's what happened to Canadian cyclist Genevieve Jeanson last year. For 2 races her hematocrit levels were too high and she was prohibited from racing. Normally this would indicate EPO doping, but EPO is hard to detect and she wasn't tested positive for it (although a doctor admitted to prescribing and administrating some to her) so she wasn't technically tested positive, just prevented from racing. Of course, afterwards she refused being tested once so that pretty much killed her career ... she wasn't tested positive, but the suspicions killed her reputation.
Hope this helps.