accessories is stuff like the alternator, powersteering pump, ac compressor, stuff like that it, an under drive pully is usually a pully that replaces the stock pully which inturn is a different size, therefor slowing the rotation or RPM of the pully making them easier to turn for the engine, hence MORE POWER BABY! but not that much haha
speculated to reduce engine life I don't know if its been proved, but you are basically removing you factory balancer and replacing it with a solid aluminum piece I have noticed more vibrations in my car, well I heard them all my loose heat shields started rattling so I removed them and bought an orp(off road pipe)
Underdrive and overdrive pulleys change the rate at which the pulley rotates. If you get an overdrive crank pulley, it takes less work for the engine to turn all the other pulleys connected to the belt. If you get an underdrive alternator or water pump pulley, you are slowing the rate at which those accesories rotate, freeing up horsepower.
"Hurting your car" comes into play here. When you slow down the water pump, you're freeing up horsepower to go to the wheels, but the engine runs hotter because the water pump isn't moving as much coolant. This generally reduces the life of the engine. The same thing applies to the alternator; you free up horsepower but you lose the amount of voltage the alternator can provide to all of the electrical components in the car.
The horsepower you get from UDP isn't worth the stress you put on the engine. If you have an svt, just save up for a turbo or a supercharger, you'd end up ditching the udp later on anyway rendering a waste of money for temporary pleasure. Unless you wanted to go extreme NA but those kind of cars have no streetability whatsoever past a certain point. Just my $.02
I tend to agree with xp0s3d, though I don't think the damage is as prolific as he makes it out to be. Slowing down the water pump may cause the engine to run a little hotter, but it will make little difference on the highway. I imagine that it's possible to offset the temp difference with an upgraded radiator, but I've never looked into it. Also, the SVT's alternator is capable of handling a lot of AV upgrades by itself, so if you don't have extra power draws on your car (neons, amps, LCD screens, etc) you wouldn't have to worry about the alternator pulley doing any damage at all. Do a search for the "big 3 wiring upgrade" and you'll be set. As for the crank pulley, well that's a big debate. Personally, I wouldn't replace the dampener (which is what is on the crank pulley stock) with a regular pulley, so just leave it alone.
For the price of new parts, I don't think UDPs are worth it. If you can get some used ones for cheap, I think it wouldn't hurt in the short term. Just make sure you keep up on your regular maintenance and the car will last a long time. But like xp0s3d said, if you really want to spend your money wisely for power gains, save up for forced induction.
yea thankz well i already bout 1 so i think imma use it for a year until i supercharge and i have another question i c alot of people with nos in there focus doesnt that hurt the car jus like the udp i thought for nos not 2 hurt ur car u have 2 have a gang of upgrades and from what i c people dont have that is nos a good way 2 go with gainging alot of power in a short amount of time