It's simply one of the most fun game modes in the crucible.
For those who do not know, Rift is essentially a Crucible gamemode where both teams have a "Rift" at their base. The goal is to collect the "Spark" and dunk it (or backflip dunk it) to the enemy's Rift to score points. First team to score 20k wins.
You can read the full info on it on Destiny Wiki
Here's some gameplay footage by Datto
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