- it allows the side to find a 5-3 fit when the notrump opener has a 5-card major.
- it allows the side to find a 4-4 fit without revealing unnecessary information about the notrump openers hand to the opponents.
After 1NT by opener, responder starts by bidding 2 clubs just as in the standard Stayman convention. But the opener now responds 2 hearts or 2 spades only with a 5-card major. Otherwise the response to 2 Clubs is 2 Diamonds which says nothing about 4-card majors.
After 1NT-2♣-2♦, responder then tells opener which 4-card major he is interested in, and opener can either raise or go back to NT. Responder tells opener he has a 4-card major by bidding the other major, thus 1NT-2♣-2♦-2♥ shows 4 spades and thus 1NT-2♣-2♦-2♠ shows 4 hearts. Responder can show both 4-card majors by 1NT-2♣-2♦-2NT;.
Here is a typical Puppet Stayman auction
Note that opener has denied a 5-card major and has denied a 4-card heart suit by not raising hearts after responder bid 2♠, but opener's bidding has not revealed anything about whether he has a 4-card spade suit as he would have had to do in a standard Stayman auction. This may be an advantage during the play in 3NT.