Bridge is an enchantment that never sees the battlefield. Well, its because it's useless on the battlefield. To make its triggered abilities work (Bridge has two of them) the card needs to be in the graveyard. Both triggered abilities the Bridge has are conditional (with an intervening “if” clause). This means that these abilities check if the Bridge is in the graveyard both when they trigger and resolve. On resolution, if the Bridge is no longer in the graveyard, the ability is removed from the stack.
Let’s assume you have two apples in the pocket you have two Bridge from Below in your graveyard. You cast Dread Return for Flashback and sacrifice 3 Narcomoeba. 6 abilities trigger. In response to Dread Return your cunning opponent activates Relic of Progenitus’s second ability that exiles all graveyards on resolution. Each trigger on the stack checks if Bridge is in the graveyard in the first place during resolution. Unfortunately, they are not there, and you don’t get a single Zombie. The cunning opponent malignantly rubs his hands.
Both Bride from Below triggered abilities are so-called leave-the-battlefield triggers. They check and refer to the game state as it was right before the object has left the battlefield.
You play the Drege deck and have collected several Bridges in the graveyard already. You cunning opponent installs Yixilid Jailer whose own static ability makes Bridges lose all of their own abilities. Fortunately, you have Darkblast in hand that you cast without hesitation. The Jailer dies, so what happens next? Are we saying goodbye to the Bridges?
No. Right before the Jailer was put in the graveyard, your Bridges didn’t have any abilities, so there was nothing that could trigger.
Bridge from Below first ability triggers when any non-token creature is put in your graveyard from the battlefield. Three things are important here:
The creature is moved between the battlefield and the graveyard exclusively, none other route the card has taken does not make the ability trigger.
The creature goes straight to your graveyard no matter who controlled it when it died.
2/2 Zombie creature token that was sacrificed for Cabal Therapy’s Flashback simply goes to the graveyard (where it ceases to exist the next time SBA are performed). Bridge’s first trigger ignores this event.
Creature cards that are put into the graveyard during Dredge, do not cause Bridge from Below ability to trigger, because these cards have never been creatures that died.
Hostile Leyline of the Void exile the cards of dying creatures. They never appear in the graveyard, so cannot trigger the Bridge’s ability. This is the reason Leylines are often in the sideboard against Dredge decks.
Bridge from Below second ability triggers waits for a creature to arrive to the opponent’s graveyard. This ability is ignorant to the following:
Who controlled the creature in question;
Whether it continues to be a creature in the graveyard;
Is this creature a token or not.
Let’s consider that you play a Dredge deck and you have come across a cunning, but not very familiar with the rules, opponent that plays Reanimator deck. Ichorid rests in your graveyard along with the Bridge. Opponent casts Animate dead and grabs Ichorid under control. At the end of his turn, Ichorid’s ability triggers and Ichorid is sacrificed on its resolution. As dying creatures can only go to the owners’ graveyards, Bridge’s first ability triggers and if nothing else happens, you get a Zombie token.
Animated Nexus controlled by an opponent will go his or her graveyard upon death and will trigger the Bridge’s second ability although it is a land card in the graveyard. What is important is that it was a creature on the battlefield.
A token that dies and is put to the opponent’s graveyard also triggers your Bridges.
Be vigilant if you control Leyline of the Void. The Leyline only affects the cards that go to the opponent’s graveyard. Creature tokens are not cards and go the graveyard, not to exile. This event will cause your Bridge’s second ability to trigger.
The Bridge in the graveyard has no controller, however, its triggers have. The controller of the triggers is the owner of the Bridges. This means that it’s the owner who will choose the order Bridge’s triggers are put on the stack.
Note, that both triggered abilities are controlled by the Bridge’s owner. This means that in case his and his opponent’s creatures die simultaneously, whether the Bridge player gets tokens or not will only depend on how well he understands the concept of the stack.
If your Ichorid meets an enemy Lord of Atlantis in battle and both creatures die in the result, no matter who’s the active player, you are the one to choose the order of triggers. To get a token you have to resolve the Bridge’s first ability first, this means that the scond ability should be put on the stack first.