The above picture shows a loss of lumbar curve, weight forward on the toes, and it's hard to tell from this angle but maybe the right knee buckling in. This is a break down in proper squat mechanics (photo taken from CrossFitOneWorld.com)
Intensity is what CrossFit is known for. It's what creates all the favorable adaptations to training that we are looking for: increased strength, stamina, speed, fat loss. Whatever our goal, intensity in training is what will get us there.
Intensity is relative to each individual, however. What is intense to me may be a walk in the park to someone else. Intensity can also be a double edged sword. We should not look to ramp up intensity before we are ready because that could result in injury.
One CrossFit mantra frequently repeated in its literature is "Mechanics, Consistency, and then Intensity". What this means is that proper mechanics come first. Being consistent with the mechanics is second. Once consistency is achieved, then intensity can safely be introduced.
Take the air squat for example. It's the most basic movement we do, but there is a lot going on in order to achieve proper mechanics. Let's say you achieve the perfect air squat: weight on heels, good lumbar curve, chest up, proper depth, knees tracking over the toes, etc. You can even get a set of ten identical, perfect squats. Then you introduce the Tabata squat. Round 1 is good and you knock off 20 squats. Round 2 is okay, but you still get your 20 reps. On round 3 you start to lean forward quite a bit, but you will not allow yourself to get less than 20! On round 4 your lumbar curve is non-existent, but it's 20 reps or bust. On round 5 your knees buckle inward, but whatever it takes, right? On round 6 you're barely doing quarter squats, but at least the reps are there. Rounds 7 and 8 you are basically collapsing and then standing back up with no recognizable form, just chasing that number.
What happened in this example was trying for too much intensity before consistency was achieved. Do that too many times and not only are you setting yourself for injury, but you are teaching your body improper movement patterns. A better option would be to back off the intensity and hit 12 to 15 perfect squats each round. The next time shoot for 1 more each round, as long as the proper mechanics are there.
The same principle applies to the SWOD's. If you are not hitting proper depth on your weighted back squats for example (the crease of the hip below the knee), then the weight is too heavy. Back off until you can hit proper depth and then build up from there, only increasing the load on the bar when the reps are completed with proper mechanics.
For an excellent article on the above concept, as well as TONS of good reading material for your rest day, check out this CrossFit Journal Article.