Okay! Due to Yuhang's request, I decided to post the rulings up for [Heart Of The Underdog] for all the players to take a look! The rulings are taken from upperdeck ya! Picture and effect below for reference.
[Heart Of The Underdog]
Continuous magic card
During your draw phase, if you draw a Normal monster card(s). You can draw 1 more card by showing it to your opponent.
Missing the Timing: Suppose you have 3 copies of [Heart of the Underdog] on the field, and draw a Normal Monster Card during your Draw Phase. All 3 copies of [Heart of the Underdog]immediately activate their effects, and because they activate simultaneously, they form a chain with Chain Links 1, 2, and 3. If you draw a Normal Monster card for Chain Link 3 or Chain Link 2, your copies of [Heart of the Underdog] do not activate again. This is because they are “when… you can” optional Trigger Effects, and you “miss the timing” because you drew the Normal Monster Card during a chain and not as Chain Link 1. However, if you draw a Normal Monster Card for Chain Link 1, all 3 copies of [Heart of the Underdog] will activate their effects again.
[Heart of the Underdog] doesn’t only activate if you draw a Normal Monster Card for your standard draw. It will also activate its effect if you draw a Normal Monster Card with a card effect such as [Reload], [Jar of Greed], or [Heart of the Underdog].
When you draw a Normal Monster Card, if you want to activate the effect of [Heart of the Underdog] you have to show it to your opponent immediately. You can’t do anything else in between.
A Normal Monster Card means a yellow card with no effect. A Ritual Monster with no effect is not a Normal Monster Card. A card like [Blue-Eyes Toon Dragon] is not a Normal Monster Card.
When you show a Normal Monster Card to your opponent to activate [Heart of the Underdog], your opponent can chain [Drop Off]. You will have to discard that Normal Monster Card, but [Heart of the Underdog] still resolves its effect.
If you draw multiple cards at the same time during your Draw Phase, such as with [Reload], only 1 of the cards needs to be a Normal Monster to trigger [Heart of the Underdog], since they are all drawn at the same time. If you activate [Reload] in your Draw Phase and draw 5 Normal Monster Cards, your [Heart of the Underdog] only activates 1 time because it says “card(s)”.
Again rulings are taken from upperdeck. To Yuhang: Hope this helps!