Have you ever started to train a large primary muscle only to have an small accessory muscle tire out early and ruin your training session?  

Have you still had gas in the tank to keep training your pecs but your triceps are too tired to continue?  

Or perhaps have your biceps fatigued before you could get the most out of your lats on back day? 

There is a simple solution to this problem that I like to call pre-exhaust.  It is when you perform a single joint exercise to isolate one large muscle first before you move on to multi joint exercises.

For chest day, you would start out with a single joint exercise such as either flies or the pec deck to isolate your pecs.  After your pecs are pumped you can move on to multi joint exercises like bench press or push ups without worrying about your triceps tiring too early.

For back day, you would start with a single joint exercise like either cable pull overs or back flies to isolate your lats.  Next you can move to bent over rows, pull ups, or lat pull downs knowing that your training session will be limited by your lats and not your biceps.