In general they are used to monitor boost levels. But you could also use it to monitor vacuum leaks, IM effeciency, etc.

They can be put on a teg.

 

great, thanks!

 

what steps can i take to see if i have any vacuum leaks?

 

You would hear it probably, listen for a hissing sound. Check your hoses, make sure the are all on and not cracked. The car will run like crap usually above 3K. If the car stutters when accelerating it's a good sign a hose has come loose.