The simplest twinkle lights sold in the stores have the entire series string go on and off. How fast they flash depends on one of the lamps (bulbs) that has a heat sensitive switch inside.
Next come the two way, three way, etc. series strings where every other or every third etc. lamp flashes together with others in its group.
Next come the two way, three way, etc. series strings with a control box where you can choose a pattern of flashing, which may or may not include fast or slow.
Depending on how the lights are arranged on the tree you may get a more pleasing twinkling appearance.
The most expensive twinkle lights (both to buy and to run) have each lamp flash independently of the rest . Most of these have relatively large lamps in parallel strings and that use all 115 volts and about 7 watts each. A few such sets, also expensive, are miniature series strings and most of the lamps twinkle independently but a few just get brighter and dimmer. You must have the proper number of non-twinkling lights in the set at all times otherwise the lamps burn out more quickly. Also if you have such a set, do not interchange the twinkling lamps with other sets.
Look at the lamps up close. If the lighted filament is a V or a W shape, the string is a parallel string. If the lighted filament is just a pinpoint, the string is a series string.
You can still buy parallel strings, usually with non-twinkling lamps, generally referred to as C7 or C7-1/2, and buy twinkling lamps separately to substitute.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm
