When you use Brace, you gain temporary HP in multiples of 1d10 instead of 1d6. All the same rules regarding the temporary HP from Brace still apply. In addition, using Brace causes you to deboost all damage you take as long as you remain stationary.
Note: Any extra damage dice rolled due to a boost or critical hit are added before the deboost from Improved Brace takes them away.