Anyone who's ever played a Fender Stratocaster (or a copy) is familiar with the basic three-spring vibrato bridge (most commonly known as a 'whammy bar'). However, what many folks don't know is that these setups are easy to tailor to suit the requirements and tastes of each musician. Tweaking a Strat-style whammy bar is simpler than most people think, and the only tools required are small and medium Philips screwdrivers and a pair of needle-nose pliers.
First, lay the guitar face down on a soft surface and, using the small Philips screwdriver, remove the six small screws from the rectangular plate covering the spring chamber, making sure to keep the screws someplace safe. Inside the chamber, you will notice three springs connecting the spring anchor towards the neck to the heavy brass tailpiece through which the strings are fed. Using the needle-nose pliers, remove these springs one at a time by securely grabbing the looped end (towards the neck) from the anchor by carefully lifting the loop up and off of its hook. Next, remove the right-angled end from the tailpiece. Repeat the process with each spring.
Now, notice that the anchor is fastened to the body by two larger Philips screws. Turning them clockwise will move the anchor closer to the body, resulting in tighter springs and a firmer feel to the whammy bar. Turning them counterclockwise will move the anchor farther from the body, resulting in looser springs and a softer bar action. Be sure to always keep the length of the two screws as even as possible. Reinstall the springs by first inserting the straight end into the proper hole in the tailpiece and then, using the needle-nose pliers, firmly grip the looped end and carefully stretch the spring towards the anchor and fasten it to the proper hook (installing the center spring first).
As you have already noticed, there are (usually) three springs inside the vibrato cavity, but enough hooks and holes to accommodate five springs. Adding more springs will give the whammy a much firmer feel, holding the bridge much tighter against the body (almost like the 'hardtail' bridges found on Telecasters). Likewise, extra springs will allow the guitarist to use heavier gauge strings, which are not only louder, but have a harder, 'grittier' feel which offers the player more resistance.
Removing a spring will increase the sensitivity. This setup will have almost a 'floaty' feel under the player's pick hand, which allows the player to use much lighter gauge strings, which will allow much more radical bends and 'dive bombing'. In either case, be sure to install the springs to where they all exert even tension upon the tailpiece, preferably by hooking them closer together at the center of the anchor and spreading them evenly apart at the tailpiece.
Finally, these springs can also reverberate sympathetically within the cavity of the instrument as the strings are plucked or strummed. This doesn't bother many players, but it can be a nuisance to some. An old trick to eliminate this problem is to simply play the instrument with the spring chamber cover plate removed, allowing your belly to contact the springs and stop them from resonating.