Workouts 1,2 means the first and second time you run through that lifting routine (3,4 are the third and fourth time, etc.; you'll do workout A and workout B a total of 8 times each)

So, for your first and second time through workout A, you're going to do the following:
A group:
15 squats (set 1)
15 squats (set 2)

B group (alternate between 2 exercises):
B1: 15 pushups (set 1)
B2: 15 seated rows (set 1)
B1: 15 pushups (set 2)
B2: 15 seated rows (set 2)

C group (alternating again):
C1: 15 step-ups on each leg (set 1)
C2: 8 prone jackknives (set 1)
C1: 15 step-ups on each leg (set 2)
C2: 8 prone jackknives (set 2)

You'll do the same thing (basically) for workout B, just use the same pattern for that list of exercises. Then, the third and fourth time you do workout A or workout B, you only do 12 reps instead of 15 for the squats, pushups, rows, and step-ups, and 10 prone jackknives.

The overall idea for the exercises that you do fewer reps over time (start with 15, then down to 12, then down to 10, etc.) is that you should be increasing either (a) the amount of weight you use to do the exercise, or (b) the level of difficulty of the exercise (i.e. you might start with 45-degree angled pushups, but then move to 30-degree, then to the floor as you get stronger) over time. For things like the prone jackknives, which only use your body's weight, you're aiming to complete more reps as you get stronger (so you start by doing 8 at a time, then move up to 10, then 12, etc.)

This should clear things up:

https://docs.google.com/spreadsheet/ccc?key=0Agkjz5fA5QX_dFV6SEtpSXB0S0NCTVpqV1JCRFhIUXc#gid=11

Tari_D: I think you're supposed to do the hardest variation that you can and still complete the reps. If bodyweight is all you can manage and still finish two sets of 15 reps (for the first run through workout A, for example), then start there. If you can do barbell squats (even if it's just with an empty bar), then you could start there instead. Of course if you don't have a bar, you could do dumbbell squats instead.

If you can easily finish the number of reps and sets you're supposed to do, then you should definitely move up to a harder variation. You want to be doing a variation or adding enough weight so that *finishing* all of the sets is hard, but you can still do it with good form (you don't want to injure yourself, but you do want to challenge yourself).

Yeah, I think that, as long as you're using good form and challenging yourself, it's absolutely fine to use whatever variation/weight you're comfortable with. And I think that improvising based on the equipment you have available is also just fine, as long as you're doing *some* variation of the exercise you're supposed to be doing at a level that is challenging (that is, I think it's probably better to improvise and use what you can find to get the workout done as close to the intended workout plan as possible, instead of just skipping exercises because you don't have precisely the "right" equipment).

For the prone jackknives (they're really difficult), I think it's fine if you can't complete the recommended number right away. I've lifted weights on and off for several years, and I had to work *really* hard to do full sets of jackknives without falling off my ball out of sheer exhaustion, lol, so I'd say just do as many as you can and *try* to increase that number as you move forward. My first time through each "level" of the workouts past the first one (i.e. when it calls for 10 jackknives, then 12, then 15) I think I was under the mark, but by the end of the second time through at that number of reps I finally did as many as I was supposed to do. I've also read that some people add a few extra lifting sessions to the end of stage one - maybe go through workout A and B each another 1-3 times, or sometimes even repeat the whole stage - just to build extra strength and endurance before moving on to stage 2...there's definitely nothing wrong with taking extra time to get to where you want to be with this stuff (and taking extra time is definitely better than hurting yourself and having to take a break).

Whether or not you choose do additional workouts, I think if you're honestly pushing yourself to do as many jackknives as you can (or pushups, or whatever), you're still working the target muscles until you can't work them any more, so you *will* still get stronger, even if you don't quite hit the "goal" for that set.