Why we use a breastplate – Some riders use a breastplate because ‘everyone else does’. Others like the security and stability wearing one gives them, as well as those riders who require one to help prevent the saddle slipping back or rotating in an accident.
There are a few different types of breastplates to choose from  with numerous variations of each, these are:
Fitting your breastplate correctly is imperative if its not done properly it can make your saddle move.
If your saddle starts to move forward
If your saddle moves backwards 
** Important point to consider is that width and fit of a saddle aside, even the best fitting tree/seat will not work **
Where it can go wrong 
Look at the girth line on this horse. This is the angle the girth must come out of the saddle. The horse has no barrel and the shoulders and muscles in front of the line will push the girth and saddle back. The girth should be built in the saddle on the angle and the breastplate hold it in that position. As a rule of thumb the girth goes from the back of the pecs, to the middle of the back unless there is a barrel preventing it.
If the girth came out of the saddle further forward or more vertically it will move to this position thus changing position of rider and balance of the saddle and ultimately the horse’s performance.
This horse will send a saddle backwards. The breastplate is fitted to allow the saddle to sit where the horse’s body naturally wants it to be and where the rider needs to be. You can see that the breastplate being used is attached to the girth between the legs and at the sides – NOT on the D’s at the top.
You can see he has full shoulder movement. Every time he lifts his front end the side straps secure the back of the saddle and helps prevent it pinching behind the shoulder
This shows that if the attachment between the legs is too loose, the girth will naturally slide back. This will then affect the saddle and the balance of the saddle.
This shows you how a badly fitted breastplate can make a well fitted saddle tight and unbalanced.
This is a well fitted breastplate if you can slide a flat hand underneath , it does fit snuggly but if it was any looser the saddle will go back until it can go no further.
Video of modified breastplate below 
** Please remember ** If your are unsure if the breastplate fits correctly. We would advise you to seek help from a knowledgeable equestrian person such as your instructor or saddler.