If you brake after the rectifier you must make sure you don't put a short across the battery.
Some machines use a change over switch but this leaves the mill open circuit during the change over. It should not be a problem but it does make life more difficult for the switch and dc switches are expensive .
Hugh suggests that you use a blocking diode in the battery line to prevent the battery feeding back into the switch. by using a double pole switch you can short the diode out in normal use to avoid the volt drop and it is only inserted as the switch changes over. This seems a better idea and is probably what you refer to.
You should be able to follow the diagram if you bear this in mind, the diode is in series with the line and you can short the mill side of the diode without shorting the battery as the diode blocks the current. One pole of the switch does the short for braking and the other pole shorts the diode in normal use.
If you brake on the ac side before your rectifier you need a 2 pole switch to short all 3 ac leads.
flux