Turboannexation is a word EU2 players coined to describe "annexing" a country via its government collapsing. It is not a true annexation: not all provinces can be taken. However, when an enemy government collapses, a belligerent country can take any number of its provinces, minus its capital and usually one other province. Generally, turboannexation is a planned result, but not always.
A 100% warscore is enough to take all the provinces of a small to medium-sized country, up to maybe 8 provinces. However, a large country can have provinces worth hundreds of warscore - for example, in 1419 China has about 500% warscore worth of provinces. To fully take over a large country via wars and diplomatic peaces can require many wars. This takes time: several years per war, usually also truces of five years in between. For example conquering 1419 China requires about 6 wars or so, so the minimum time you can take them out via traditional wars is perhaps 40 years.
By contrast, turbo annexation is very fast. It requires one well-planned war, plus one or two easy "mop up" wars against the rump state that is left after a turboannex.
Turboannexation and Badboy
Turboannexation costs 2BB per province taken. As such, even doing it against a pretty small country is likely to rack up huge BB costs. Thus, generally it's something you avoid, except when you've already broken the badboy limit and are "going for broke". On very hard, you'll be in badboy wars.
How does a player plan a turboannexation? Sometimes, you can't. However, there are several circumstances where it's pretty easy. Basically you need a guarantee (or at least to have good odds) that rebels will capture one enemy province.
The most predictable way is when you've got an target country that has already had a rebellion in one or more provinces. If you DoW and invade quickly, you can kill all of the enemy's armies such that he can no longer attack the rebels and/or siege the province. Now you can siege out the enemy. Note that if the province can revolt or defect, you must siege quickly enough (within 3-4 years) to cause the government to collapse before the province can escape. Consult revolt.txt, or siege quickly.
Another reliable circumstance to plan a turboannex is when the target country has high revolt risk from a scripted event. If you know (usually by reading events files) that the RR will continue for a while, you can invade the target and start sieges, waiting for rebels to appear somewhere - anywhere. Don't complete any sieges until a province has a rebellion. When rebels do appear, you retreat, leaving them your siege.
Yet another way to go, though not guaranteed, is to invade when the target country has started a religious conversion attempt. This is not guaranteed - the conversion might succeed or it might not. If it fails, you'll have the rebels you need. Obviously, you want to not complete the siege on the province being converted; just cover it and wait to hand it off to the rebels.
If you read the target country's scripted events file, you may be able to exploit historical events with revolt commands. If they are random rebellions (targeting -1, meaning, not a specific province), you can complete all sieges except for one province (preferably the capital). When the event fires, it will only target currently controlled provinces. Thus it will necessarily hit the one province you've left unconquered, and you can then grab everything else.
Another possibility is to use rebels from a neighboring province. (Typically this will be a province you own.) Suppose you own an isolated province, whose only adjacent provinces are owned by the country you want to turbo-annex. If your province is of a religion you can afford to repress, drop the tolerance slider to zero. Eventually a rebellion will happen; the rebels will siege your province, take it, then move on to the target country. (You can force a rebellion when you want it by inducing rebellion checks via moving your tolerance sliders and closing the tolerance window.)
One final way to sometimes get turboannexation is to simply attack with no planned rebels at all. Capture all provinces but the enemy capital, which you can siege for a while but then just cover. If you're lucky, the enemy will get a rebellion as a random event. Sometimes your own war exhaustion will reach a critical level, and then you'll want to make peace. You can complete the capital siege in a few months and make a "normal" 100% warscore peace. Then you return in five years to try again. But you can wait for years if necessary.