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)”.