I think its fairly obvious. Don't know why people are saying its bullshit
Exercise 1 uses muscles A-B, your weak point in that lift is B
Exercise 2 uses muscles B-C , increasing your strength in exercise 2 increases your strength in exercise 1, but your weak point is muscle C so you can't effectively work muscle B with the exercise.
Exercise 3 uses muscle C. Increase your strength in muscle C means you can more effectively perform exercise 2 strengthening muscle B making exercise 1 stronger.
Ill give an example, I know it obviously doesn't work this exactly in real life, but as I said its just an example.
Lets say the front squat is an upper back and quad strength exercise
You perform rows, chin ups or any other pulling exercise to build up your upper back strength, but your arm strength is lagging so you end up jerking your body to get the weight up and don't get any back involvement at all so you perform bicep iso work to get increase arm strength so you can row better.
Probably not the best example for the powerlifting section since it involves curls and front squats. Personally my bench goes up when my squat goes up, don't know why it just does, probably because it puts a bit of mass on my upper back. To get my squat up I perform lots of good mornings, RDLs and back hypers. I wouldn't say performing back hypers directly increased my bench.