If your using a SB, the line load will never exceed what the winch is capable of pulling (i.e. if you have a 9K winch, even with 5 SB's you will still not put more than 9K on the winch line). HOWEVER, the lines/cables/chains (and SB) WILL see an increase and that is where you can get into trouble.
For a simple example:
9K winch, to a SB at a tree, back to the rig. Over-all pulling speed will be 1/2, line load on the winch line will be 9K, but pulling force at the tree (and what will pull you out) can be 18K. Better be sure you have a good chain/strap.
probably was all available in that link Ben posted
For the record I carry (1) SB. But don't think I've used it in a few years. Handy when you need it, but otherwise just holds down the bottom of the winch box.
________ IO LIGHT VAPORIZER