@guardian7 said it nicely.
@Don Fairbanks had a great description of his view of density training. The reps must be "passable" form-wise or words to that effect. Lousy form reps don't cut it.
I do the 30 minute versions for 3 weeks ramping up each week with my peak at 3 weeks. I then reduce volume by 20% for each day and shoot for that number during week 4. Once I get it I quit for the day, whatever time that is.
The heavy volume or "heavy rep" day if you will to me is the same as the other days. Often I get fewer overall reps on these days because the higher reps require more recovery.
I wouldn't over think it as I don't think it really requires too much dissecting, just do the required work for each day and make a bit of progress next week! "Progress" can be the same volume done more quickly, not just more sets!
Admittedly although I'm made "progress" this week over last week, it is much slower going with the 36s than it was with the 32s.