https://upswingpoker.com/minimum-defense-frequency-vs-pot-odds/

This is a pretty good article from Upswing (free articles) that discusses this. One of the main takeaways from the article is that MDF will be used more off table unless you are playing in unknown environments or unknown opponents. I play anonymous games so I use it a bit more but honestly I use it more when I am deciding to bet to see what fold equity I need vs using it to make calls. I think pot odds are probably more effective to use for calling decisions (especially in unbalanced pools).

Using the MDF, we look at the portion of our range that we need to call in order to remain unexploitable by bluffs. Based on pot odds, we actually call the portion of hands that makes the call a winning play.

You got it.

MDF is not applicaple in all situations, it is most useful in pure bluffcatching situations on the river.

Therefore potodds and MDF often spit out kind of similar numbers, they seem to be connected somehow. But there is some huge differences.
The use of MDF is basically to prevent us from beeing exploited by specific betsizes from Villain.
Will try some dumb toy games to show that there basically is no real connection between potodds and MDF.
Imagine a HU match where we are BB. SB minopens, we flat/check.
Villain bets pot, 4x pot or goes all in for 100x pot. This is really difficult to defend as we rarely get the correct potodds to call an 100x PSB. Here MDF can be usefull as we have to defend this strategy somehow. But MDF can be pretty problematical in this sense:
Same toy game. But this time SB is only allowed to raise AA and BB is only allowed to defend 23o. Unluckily we both get these hands and face an AAK flop and after checking our nutlow SB again decides to jam 100x PSB....
So we basically have 0% equity in this spot with 23o. But MDF doesnt allow us to fold our whole range (as this would be exploitable). So basically MDF would force us to make a call in this spot with 0% equity.
Even though this is far from a realistic example, it shows some huge problems of MDF. Even though these numbers often look like sort of potodds related, they are not.
So if we play opponents that are unable to exxploit us with certain betsizes (That obviously could also be way lower than these overbets here!) we should usually just look at our potodds until the river. And even OTR in most circumstances defending below MDF will be the better play, as we will see the nuts too often when we face huge bet.
So if we face an 4x riverbet, its fine to basically fold everything but the nuts usually. Even though this would be a mistake in mdf-land.

Keep in mind that some game theory concepts like MDF really only make sense to apply in situations where we are facing opponents who are playing in a way that (either intentionally or unintentionally) is doing close enough to the right thing GTO wise. Using MDF specifically as an example, the whole purpose of it is to prevent our opponents bluffs from being profitable entirely on their own. An opponent who isn't bluffing enough in a given spot isn't something to be concerned about in this regard and as such we are better off just using pot odds vs. his range. An opponent who, on the other hand, is bluffing at least enough is someone who we will lose money to over the long run if we don't defend enough of our hands. That is when MDF would be a concern.

Balance is the main term here to take into account. Both concepts are somwehat related to indifference. As a reminder: Indifference (also known as equlibrium) is a point where neither player can switch their options to gain more EV from the other player. This is basically what all this GTO-stuff is about.
To take your example: We raise and face a 3bet of half pot. Our hand/range needs 25% potodds to make a call here.
So if we have a hand/range that has exactly 25% equity vs the 3betting-range neither of us is making/loosing money in this spot.
So imagine raising KK here and facing this 3bet and like a miracle we know villain has AA here always. We dont have 25% vs this hand. So we should be folding this every single time.
BUT if we look at the mdf (that doesnt care about ranges, equities and so on) we should be defending 66% of the times here (so 2/3 of our range or, given the example: KK 2 out of 3 times), even though we know that it is a loosing play.
MDF is heuristic for players that are playing vs balanced opponents.
So if we open a balanced range and face a 3bet from a balanced range, we can quickly assume how much we need to defend here to stay balanced.
If we play opponents that are heavilly unbalanced (ie ususally plaing too much value mostly) MDF gets pretty useless/dangerous.
So if we face a 4x overbet OTR in smallstakes-poker, MDF suggests us to defend 20% of our range as an extreme example.
Im pretty sure that you will loose tons of money if you call with that frequency in this spot vs anyone at smaller stakes always.

Pot Odds based decisions kinda suck. If you have 32s in the BB, you probably have pot odds to call when the whole table comes in but its going to be -ev. So I'd lean towards minimal defense frequencies. But I don't even know if being unexploitable = GTO. Feels like some spots will always leave you exploitable if you try to max EV. It's all a grey area and very conflicting. Like calling K7s on button vs BB 3bet at low stakes. Yea you are going to call that hand to meet the minimal defense frequency but I don't even think its even +ev. When stuff like that happens I prioritize ev over frequencies.

I would simplify these concepts to two things (at least the sample I played this way when I was concerned with my redline & WWSF% worked out quite well):

1) Use MDF when deciding to be aggressive. In other words, ask yourself the question, if I, say, bet pot here, will he fold the appropriate amount of the time? Of course there are exceptions (nutted and allowing them to catch up, etc). Use this basic idea and see what you can get away with. Experiment with it.

2) Use your pot odds when deciding calls. Look at your FD's/SD's/Combo Draws and look at your direct odds and then try to figure out if you have implied odds. I see guys paying pot+ on draws that catch <25% of the time. This is massively unprofitable for them if we learn to identify when to get away on the next street when the bad card(s) hit and extract big bets from them 4 times out of 5 (again, very rough). Study how you are playing your draws passively. If you are making glaring mistakes you will see it is mostly paying a bad price/overestimating implied odds. Think about the times you paid a 2/3 pot bet with an 8 out straight draw when they are in a spot that is bluffed a ton. What good is paying a horrible price drawing to an 8 out draw when they are betting in an over bluffed spot?

whiteshark I am assuming that you are the Whiteshark that Steve P. review, is that right? Hope so, looking forward to watching it.