My interpretation/paraphrasing of a few of the weed scientist's (Dr. Clarence Swanton, University of Guelph) main points from the transcript for those that don't want to listen/read it:
Rapid and irreversible yield loss can occur when the crop detects (one method of detection  is via far red light that is reflected off the weed and sensed by the plant via phytochromes (type of protein)) a weed early in it's life; losses not explained by competition for resources (sunlight, water). This sparked several years of his research. What he found was when the plant senses a weed, the plant dramatically increases production of singlet oxygen. A little bit of singlet oxygen is tolerable, but the amount produced in when weeds are nearby is too much (too much results in cell membrane and DNA destruction). If too much is produced the plant has to produce antioxidants to try to fix it. Producing these antioxidents forces the plant to redirect energy from other things.

When the weed is sensed, the plant down regulates the production of photosynthetic genes and you get down regulation of genes that are responsible for chlorophyll biosynthesis, resulting in a loss in photosynthetic efficiency and carbon assimilation.

He also has found that when the plants sense the weed, that plant upregulates genes that diminish the pool of Jasmonic acid in the plant. Jasmonic acid helps the plant defend against insects and disease. So the plant reduces its ability to defend itself, to put more resources into growth. When this happens you see long spindly plant starts to grow.

Most recently he found that in the presence of weeds, the plants may look yellow, making you think they are short on nitrogen, but the plants are actually full of nitrogen. The nitrogen is being accumulated in the vacuoles (storage sacks) in the root, and in the vacuoles in the in the leaves. It is building up but not being moved through the rest of the plant parts that need it. It may be more than nitrogen doing this, he just hasn't researched it further than this. The plant is downregulating genes that tell the plant to move nutrients throughout the plant.

The most controversial point he made in my opinion, may be that we "have to learn to live with resistance, as opposed to defeat it, because you're not going to defeat it". This is counter to other messageing.  My interpretation: we cannot go for 100% control of the resistant weeds, its not practical. I immediately thought of this video: Resistance to Group 15 Herbicides : 

Near the end of the video: “Water hemp seed does not persist indefinitely in the seed bank on average you're probably looking at somewhere around seven to ten years and then most of the viability of that seed is gone, so how can you use that information trying to better manage these resistant populations. If a grower can do whatever it takes to ensure that at the end of the season that there's no seed produced by the female plants, if you can maintain that practice for three consecutive years four consecutive years you will quickly notice that the population begins to decline very very rapidly. That is the weakness; simply waiting for one of the herbicide manufacturers to reintroduce a new active ingredient that's effective on waterhemp that's not going to be the long-term strategy.”