The idea behind Minimum Defense Frequency is to defend enough of your range such that villain cannot profitably bluff with any two cards. Defend less than MDF and the EV of villain's bluffs become more than 0--which surely leaves us vulnerable to getting exploited?!

Not quite! There are many situations in poker where it's perfectly fine to allow your opponent to bluff profitably. This mostly happens when his bluffs have some EV as a check.

For example, imagine if the board runs out in a way that is extremely favourable for our opponent: our range is mostly bluffcatchers and missed draws, while even his worst holding has a pair. (This happens sometimes for EP vs BB, when there are multiple high cards on the board.) If EP's strong hands want to get paid off, he will need to turn some of his worst 1p hands into a bluff. Therefore, BB should overfold relative to MDF, in order to give these 1p hands some EV as a bet (otherwise they would check and take their showdown all the time).

Given the abundance of river spots where we underdefend, it's a little strange to think of MDF as "Minimum Defense Frequency". So why not think of it as "Maximum Defense Frequency" instead? Watch this video to see how MDF is derived, as well as why we very rarely exceed it on the river:

Awesome video!

I saw in another one of your videos that you should defend less than MDF on many flops BTN vs BB. Could you elaborate on why this is the case?

Because BB has a lot of hands without enough equity to profitably continue

On the flipside, IP has high check EV so needs to get overfolds to justify betting

Great video! I’ve been arguing about MDF with people for literal years. Maximum Defense Frequency is a much better way of thinking about it.

I would like to point out that there are spots overdefended spots, though they are very uncommon (ex: when there’s a straight on the board).

Yup, I mention this very briefly towards the end.