How do you compete if the opposition opens 1NT? Doubling for penalties if the opening is weak is very attractive, but over a strong NT it is better to employ the double as part of a defensive system. There are many such systems, some simple and rather ineffective, some complex and prone to errors of memory. Brozel however is efficient and relatively easy to remember.
The double indicates a single suited hand. With a balanced hand and 9-11 points, partner can pass. With a strong hand partner may respond 2NT, and the doubler then bids their long suit. Otherwise the response is 2 Clubs, after which the doubler bids their long suit (or passes if clubs was the suit); this response can be at the three level with a strong hand.
Brozel can show two-suited hands as it accomodates Pinpoint Astro, where any two level bid over 1NT shows a two suited hand. Hearts is the pivotal suit, so:
2 clubs = hearts and clubs
2 diamonds = hearts and diamonds
2 hearts = hearts and spades
2 spades = spades and a minor
2NT = both minors
A 5/5 distribution is required, or all the honours in a four card suit. Partner can support a known suit, or over 2 spades bid 3 clubs for pass or correct.
As an embellishment, overcalling 1 NT with a three level bid shows a three suited hand with the suit bid being the short suit.
Also, the double can be used over the opposition's response to 1NT, with the same responses as above.