Stop and Alert are different. Before bidding boxes, we used to say, "Skip bid, please wait," which was asking the next bidder to wait ten seconds before acting following a jump bid, which is what you described.
Alert means that a bid doesn't have the commonest meaning, which is usually natural. Thus, if partner opens 1C, we respond 1S with 5 spades and 11 high card points (HCPs) and opener rebids 1NT, a 2D bid there is artificial and asks partner to describe her majors suits (spades and hearts) more and says nothing about diamonds, so the bid must be alerted.
Here's the convention if you'd like to look at it. It's very useful for hands where responder has a 5-card major. In an auction that begins 1m-1M (that means an opening bid in either minor and a response in either major. 1C/D-1H/S only promises 4 cards in the major for responder, and we are looking for 8+-card major suit fits, which are difficult to uncover when opener has 3 cards in responder's 5-card major without a convention like this. Moreover, responder and opener may have 4 hearts each if the bidding has proceeded 1m-1S when responder has 5+ spades and 4 hearts:
New Minor Forcing Bridge Convention - Bidding and Responses
Here's a hand from the link. The partnership has a 5-3 spade fit and a 4-4 heart fit. How shall they know that? The 2C rebid by responder is the New Minor Forcing convention (partner opener opened in clubs, so 2D is the alertable bid; had opener opened 1D, NMF is invoked by bidding 2D, the unbid or new minor) and asks opener to bid hearts if he has four, and failing that, to bid spades with three.
This hand should be played in hearts. Why? Because when one has a choice between a 5-3 fit (spades in this case) and a 4-4 fit (hearts), 4-4 is the better contract. Why? Because missing 5 cards in each suit, the opponents' cards are most likely divided 3-2, meaning it takes three rounds of trumps to pull them all. Playing in hearts, we have a trump left both in hand and dummy, which opens up the possibility of a strip-and-end play and also allows discarding on the spades. We can't do either of those if we play this in 4S.
View attachment 96869