About HIT itself . . .
If you're a fan of football, it actually helps to look at bodybuilding as a 60-minute game. Progressive strength training for building muscle becomes a game of gaining as many yards as possible within that time frame. Variables become such --
1) Rate of muscle gaining "ability" is the equivalent of the raw speed of your offensive players.
2) Your relative training level is the reflection of the raw speed of the opposing team.
3) Your ability to rebound from CNS training stress and improve skill acquisition is the equivalent of how good your offensive line is.
4) Frequency is the equivalent to completion percentage.
HIT/HG, because they emphasize failure training and a fairly linear progression scheme, are basically vertical offenses. They're about generating a lot of yards per play to the possible detriment of completion percentage.
In the Mentzer/Little school of HIT, they emphasize long post routes with a very low completion rate (very low frequency.) It's maybe the most vertical offense there is. As long as you can continue to generate not only yards per play (strength gains), but very significant yards per play, you'll see good results. Having a good offensive line (i.e. good neural recovery rate) helps a lot. Again, you have to accept that you get less chances to stimulate gains, and so you have to make it up by pushing forward significantly every time. Your line (your recovery rate) needs to be healthy and well-rested.
In the Mentzer school of thought, basically, the reason why your completion rate goes down is because the defensive teams get faster and faster at higher levels (reaching the "genetic limit.) Your offensive line can only get so much better; your team speed doesn't really improve. Thus, in an extreme situation, you may, by Mentzerian necessity, find yourself only training a body part once every 3-4 weeks, the equivalent of basically throwing 4 or 5 hail marys per entire game to score all of your points (muscle.)
High risk / high reward. The key to success in Mentzerian HIT is to throw longer and longer pass routes, to keep challenging yourself with risky increases in training load. And to accept the long periods of inactivity, readying yourself for the next opportunity to go long. It happens to be when you've fallen behind by a lot of points (i.e. long layoff), or when the opposing defenses aren't fast (i.e. beginner level), the significant advantages in your offensive line (well rested CNS) and team speed (muscular development) can enable you to be extremely successful with this scheme.
Most of the HIT/HG schemes are not this extreme.
1) The McRoberts cycling wrinkle is the equivalent of balancing shorter (but still long) slant routes with a regular, if predictable, running game (i.e. lesser load, non-failure training.) This scheme is optimal when the balance of run and pass eases the demands on the offensive line and enables a high completion rate (i.e. 2x week of steady strength progression.)
2) Max-OT is the equivalent of the hurry-up, no huddle offense. You throw long routes to your fastest guys (i.e. choose exercises of highest dividends and work in a powerlifter's load range) in the hope that the defense will not be able to adjust until you've already accumulated a lot of yards. You accept that there will be lengthy droughts (i.e. the layoff period), just as long as there will be periods where you can catch the other team off guard and assault with a lot of deep throws.
3) Darden/Jones programs presume that your completion percentage will be very high, and give you a lengthy, lengthy playbook with many passing patterns and misdirection plays. However, you still never run the ball. Their preference of machines is the equivalent of your offense running indoors and on artificial turf. Although not required, this leverages a significant advantage with your overall team speed over the defense. That is, it can take longer to plateau. The hope is that, with all of the possible high-risk choices at your disposal, you can find something that you can complete at a high percentage. It's arguably the most demanding of all.
4) IART-influenced HIT schemes, arguably the most advanced of the lot, takes Mentzerian HIT to the next level. The offense can be very, very vertical, but they also include the equivalent of play scripts (Chaos training) and the exact measurement/timing of routes (TUT) to maximize the efficiency and yard potential of each slant, post, and tight end play. Basically, they're to HIT what the Don Coryell/Mike Martz system is to vertical offenses. They're also, arguably, the only HIT variation that examines the issue of ball possession time (i.e. the amount of time per year where the body is actually growing.)
5) Finally, there's DC training. DC isn't HIT/HG per se, but a lot of HIT-ish elements end up in there. I'm not sure how to describe in football terms, except that it's probably the closest thing to USC Football. Everybody discovers their inner Reggie Bush with this system.
As most football people know, most NFL teams don't use vertical schemes. And, while yards per play is important, their principal means of scoring a lot of points is securing ball control and ball posession time. That, regardless of the speed of the opposing defense, the key is to run and execute the right play in order to eventually move forward and keep the offense on the field.
That is where periodization schemes come in. They are the equivalent of the West Coast Offense, a game of read-and-react, give-and-take, and experience, in order to push the line forward regularly and with a predictable level of control.
They look at the vertical offenses as impractical and grossly suboptimal at the most demanding levels, somewhat limited, if not condescending, to the trainee's advanced experience. They cite non-biased empirical/statistical evidence that proves the superiority of conservative, frequent, and dynamic scheming over the high-risk sameness of HIT. In periodization, an offensive line (recovery) can be schemed carefully (Dual Factor Theory) in order to push for more ambitious gains per yards.
In contrast, proponents of vertical HIT see the overweening complexity of advance periodization as disattracting, unproductive, and too probabilistic (wimpy, even) against the "certainty" and immediate reward of a completed long pass, and the apparent realities of the team's speed. And they always cite the Favre-ian comeback, a recent instance of really big personal gains, as proof of HIT's worth.
A smoothly run periodization scheme is like the 80s Montana or 90s Elway teams. A badly run one is the aimless "dink and dunk" of Steve Mariucci. A smoothly running HIT-esque scheme is USC running up points on UCLA. A failing HIT-esque scheme are the Norv Turner Raiders throwing interception after interception as Kerry Collins forces passes to Randy Moss.
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