Source/Fuente: southamptonbridgeclub.blogspot.co.uk by David Huggett 
It is dangerous to overcall a 1NT opening on a limited hand, say 11-14 points, with a 5-3-3-2 distribution because you are very much open to a penalty double by the partner of the 1NT opener. He will know that his partner not only has a balanced hand which makes trick taking more likely but also that he has at least two trumps. However if the overcaller has a two-suited hand – and by that I mean at least nine cards in two suits – then as long as he can show those two suits he is in a better position because there is twice as likely to be a fit with partner.
Many schemes have been devised but the easiest to learn is ASPTRO and it works like this:
1) 2 over 1NT shows at least nine cards in HEARTS and another.
2) 2 over 1NT shows at least nine cards in SPADES and another.
3) 2/2 are natural, 11-14, with a six card suit.
4) With both majors anchor to the longer major. i.e. bid 2 with four spades and five hearts.
The responder to the overcaller usually wants to settle in the best partscore so if he has at least three cards in partner’s known suit he bids that suit at the appropriate level, but beware that the overcaller may only have a four card suit.
Example:
With: K x x A x x Q x x x x x x
bid 2 if partner overcalls 1NT with 2.
If the partner of the overcaller holds less than three cards in the known suit but with a limited hand then he bids the next suit up i.e. 2 over 2 and 2 over 2. The overcaller will either bid his known suit with five or his other suit if he has only four of the known suit. If the overcaller bids his known suit and the responder has a singleton or void then he bids 2NT to get the overcaller to bid his five card suit.
Examples:
With: x K x x x A x x x x x x x
bid 2 over 2 and if opener now bids 2 bid 2NT for the overcaller’s second suit.
Examples:
With: K x x x K x A x x x x x x
bid 4 after partner overcalls 1NT with 2 with this seven loser hand.
Things to note:
1) An immediate bid of 2NT by responder is natural and shows 11-12 points.
2) The only way responder can make a game-forcing bid is by raising the 2/2 cipher.