prince - A suspension is a total package. Both the springs and shocks work to control compession (bound) and rebound. This is an illustrative example that is light on technology, but hopefully conceptually helpful.
In the simplest of terms, think of a spring - not like the one on your car, but a little one that you can compress w/ two fingers. When you compress it, you can easily feel how it takes force to compress it. When you release it, it "springs" - what some would call bouncing. This is b/c there is no force to prevent the spring from extending to its full lenght ASAP. Apply that to your car, only on a much larger scale <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt='
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The next piece is a shock absorber (hey... the recoil of a spring is like a shock... can that be absorbed? <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/wink.gif" border="0" valign="absmiddle" alt='
'> ). If you've ever played with a shock, you know that your bare hands can compress the shock and help it to re-extend (rebound). Unlike the springs on a car, it is movable - just at a very slow rate. Now, think about how this would affect the springs... While the spring requires force to compresss (waaay more than the shock), the shock removes the "boing" from the spring as it is much slower to rebound (waaay slower than the spring).
Now, as for that total performance system, there are a couple other characteristics - most importantly though - suspension travel and spring rates. Factory specs give you about 3", IIRC and you lessen it for every bit lower your car is. Less than an inch make for an ugly ride AND blown shocks b/c the limited amount of travel forces them to bottom frequently. Spring rates work in a similar way - the higher the rate, the more force the spring can take for compression, but the more explosive the rebound. A corrollary problem of this is that a cheap shock can't handle (or "damper" or "absorb") this rebound and extends too fast and too far (bouncy).
The simple rules from this are:
1) you can go too low = run out of suspension travel
2) you can go too low for the rate of your spring (IMHO, the problem w/ Sportlines) = too little travel, not enough spring to stop bottoming
3) you can go too stiff = too much rebound for weak shocks
4) you can go just right = matching spring rates with shock valving and adjusting rebound for the appropriate conditions.
Note: this is only a rough explanation of what I thought were important points. It is not complete or technical - just basic. Add what you need to if you think it needs more <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt=''>