Not for nothing, but that's 455 pounds.
First off, what's the difference between a "complex" and a "combo". As I know the terms, a complex is a series of exercises down sequentially for a given number of reps. Example: deadlift, hang power clean, push press, front squat, RDL, bent over row for 8 reps each. These are done for speed and generally for a metabolic conditioning effect; the reps are also too high to allow you to work on technique. A combo is several exercises done sequentially for a prescribed number of reps...sound familiar? The example here would be: clean pull, power clean, 2 push press, 1 jerk, with the purpose being to work on a particular weakness during a lift. I suppose its a small difference, but this is how I define the differences when I talk to someone about it.
I think the big difference is, indeed, in the purpose of the prescription. A weightlifting combo should be used to over-emphasize one aspect of the lift (snatch or clean and jerk) while under-emphasizing another aspect. It also allows you to get in more volume in that lift, with a specific weight and specific positions for a specific training effect.
Lets use the Klokov Complex from above as our example. Clean pull + clean + paused front squat + push press + paused jerk. In one of his american seminars someone asked Dmitry why he did so many deadlifts and presses and he said that it was because those are his weakness. This gives us some insight into the design of this complex. Since he wants to work on pulls, he gets in two of them (the pull and the clean). Since 205 is a "light" front squat for him (he did a 5-second paused front squat with 250kg) he adds in a slight pause to increase the difficulty. He challenges himself with a heavy push press, and since it's a relatively light jerk for him, he adds the pause during the jerk which also reinforces his jerk dip position. This combo is very specifically designed to address his weak points and reinforce some important positions.
Combo's are appropriate for any level of lifter, but the combo itself must be appropriate for that persons ability level. Taking a lifter who's been training for 5 months and throwing the Klokov complex at them is probably a poor idea. If their front squat position is poor, that will be a shitshow. Re-racking a bar from the overhead position is a skill that takes a while to learn and a paused jerk would result in a missed lift.
A more appropriate clean and jerk combo for a beginner (with a poor bar path on their clean and a lot of missed jerks) would be something along these lines: 2 clean pulls, above the knee hang clean, front squat, jerk, behind the neck jerk. This combo would give the lifter 2 pulls to the waist where they can focus on positions. Then the hang clean would allow the lifter to focus on keeping their shoulders in front of the bar without having to worry about clearing their knees. The added front squat is because, well, front squats are awesome. The front/back jerks allow the lifter to get two jerks in without having to re-rack the bar onto their clavicle, and the second jerk is the easier variation when they are already smoked.
There are very few wrong times to use combos, in my eyes. They are very useful for building hypertrophy and work capacity at the beginning of a phase, just after a competition. They are useful for working on positions in the middle of your training cycle when you're still ironing out the kinks, so to speak. I would not, I think, use them pre-competition. That's when you should be focusing on getting comfortable handling the weights you need for your competition. They would be, however, useful during a deload week since combo's are, by nature, a lower intensity exercise.
I use combos' pretty regularly in my training/programming. Let me outline a couple of my favorite combos and why I apply them.
1 Snatch Pull + 1 Hang Snatch below the knee + 1 hang snatch above the knee
- I use this combo because one of my big issues is clearing my knees in the snatch. The pull to my knees is pretty good, but once I get to my knees shitty stuff starts to happen. This let's me work on various aspects of the first and second pulls in sequence, and I can feel where I'm messing up.
2 Hang Power Snatch + 1 Hang Snatch
- I do these above the knee; I'm really bad at hang snatches. I get my hips way back and shoulders over the bar instead of out in front; this makes me bang the bar off my hips and gives me a shitty bar path. I am able to use a lighter weight and focus on my positions and really pulling the shit out of the bar. The last full snatch lets me use that long pull I just developed and really dive under the bar without any "weightlifters disease".
2 Power Clean + 1 Jerk
- I clean more than I jerk, so I tailor my combos to reflect that usually; this is the exception. I power clean less than I full clean, so this combo allows me to work on staying over the bar and pulling hard on a lighter weight. The jerk still takes "thought" for me to do, and I can focus on getting a really solid dip out of it, rather than bench pressing it.
Clean + Push Press + Jerk
- I actually really like this combo for myself. Performing a push press before the jerk forces me to drive with my legs and (hopefully) keep my heels down. I go to my toes too early during my jerk (just like everything else) so doing my push press without lifting my heels reinforces that motor pattern.
1 Clean + 1 Jerk + 1 BTN Jerk
- I apply this combo to myself to get the feeling of higher percentages. My jerk is a shithole, so doing a jerk double with a high percentage is doomed to failure or shitty technique (same thing, right?). I skirt this issue by doing my first jerk and then dropping the bar onto my back, where I can do a BTN (behind the neck) jerk. This second jerk feels "easier"and let's me get in a second rep with that heavy weight.
As I said before, there's nothing magic about simply performing combos. You need to do the right ones at the right times in order for them to be successful. Practice doesn't make perfect; perfect practice makes perfect. Start making up some of your own, focusing on your weak points and see what happens...and don't forget to track your progress!
Let me know what you're trying and how it works for you. Have a great day and go lift some heavy shit!