You need to use “Up the Line principle” when you use 5-cards Major opening. Why? Because when you open, i.e 1♥/1♠ = 5+ cards in ♥/♠ accordingly, you will need to open with 1 minor if you have a 4-cards Major.
(Yes, true that you can have 5 or more cards Major with 1 minor opening. However, it is very specific only when you have 6-5+ 2-suiter hand which require you to bid the longer suit first.)
So, in this situation, by using “Up the Line principle” you can find your 4-4 fit in Major, especially for a part score.
In this case, the responder will prioritise to bid their 4-cards Major first, upon their longer minor – providing the responder’s hand is not a game force hand. If the responder has 12+HCP, the “longer suit first principle” still apply.
And also, “Up the Line principle” requires the responder to bid their major from lower rank Major first, i.e Heart. Bid Heart first before Spade even when you have longer Spade. This seems rather contra-intuitive, but it is important to understand.
Imagine you have this hand as responder:
♠KT542 ♥K973 ♦T98 ♣7 (5-4-3-1, 6-HCP)
When your partner opens 1♣ or 1♦, because of your limited HCP, you need to bid 1♥ first. So, if opener has a 4-cards ♥, he can then the bid raise to 2♥ and play there.
But if you bid 1♠, the opener may bid 1NT. Then you will play a more inferior contract as your partnership actually do have a 4-4 fit in ♥. (Remember, opener cannot re-bid 2♥ unless he has “reverse hand” value, i.e: ~16+ HCP)
So, this principle is quite important for you to understand it fully.
With limited hand, always bid your 4-card ♥ first before longer minor or even longer Spade.— Up The Line Principle