This is the technical part of DPOS. How the code is written to achieve consensus. The other half is how the HUMANS interacting with the code are helping to achieve consensus.

He mentions in the beginning of the video that we will just assume the block producers have already been elected and proceeds to explain the code part. Much needed!

But for those of you interested in the role, us humans have in the DPOS system, let me give you a quick explanation.

Each core asset is one vote. On Steem, it's the powered up STEEM token (VESTs) and on BitShares it's the BTS.

Users vote for block producers and their vote is proportional to their stake in core assets.
The candidates with the most votes are the ones that get to produce blocks and hence get block rewards.
Now if you are a bad blockproducer, the voters can remove their support (by removing their votes) and you will lose your position as a paid block producer, rendering you harmless.

So humans that have stake in the system (something to lose) get to vote in competent, qualified people while having the power to fire them if they misbehave and threaten the network which you have stake of.

So not only does the code's rules help to achieve consensus but also the actions of the token holders (us!) assist in defending against potential attacks.

Code by itself is not nearly as powerful as a system that has hardcoded rules AND monkeys incentivized to keep the system honest.

DPOS is the best thing out there at the moment. It's crazy that it's also the fastest and more economical! Not energy wasted (like in PoW)