Detraining, quite simply, involves taking a week or two off completely from working out.  You are giving your body the chance to recuperate its hormonal levels and are helping it overcome plateaus.  To me, it is a form of periodization because you are taking a few steps back to enable yourself to move your training forward again.

There have been several scientific studies that seem to indicate taking a rest break after a medium sized period of training time is beneficial in many ways.

Strength won't decrease

A study published by Med Sci Sports Exerc. 1993 Aug;25(8)929-35 involved 12 powerlifters who took 2 weeks off.  Their strength did not decrease significantly.  Their Type 1 (endurance) muscle fibers did not decrease.  Their Type II (hypertrophy) muscle fibers decreased by 6.4%.  Growth hormone increased 58.3% and testosterone increased 19.2%.  This is a very positive outcome for the powerlifters in question.

IGF-1 will increase

Another study by J Strength Cond Res 2007 Aug;21(3):768-75 was done on 46 men.  It compared stopping resistance training for 4 weeks to tapering it off over the same time period. The study was done after 16 weeks of consistent training.  The total break group had decreases in strength and power (9% and -17% respectively).  However, the total break group had an increase in resting levels of IGF-1.  IGF-1 is a hormone involved in muscle growth. The taper group had slight increases in strength and in IGFBP-3 resting levels.  IGFBP-3 is a protein that binds to IGF-1 to deactivate it.  Thus, the tapering group was lowering an important muscle growth hormone.

Are you truly working out with maximum intensity?

You've upped the intensity of your workout, but are you still taking some with you off the gym floor?  Are you taking your sets to complete failure or are you stopping just short of failure?  Are you lifting weights that are heavy enough to spur on changes in your body?  If you are not leaving everything on the gym floor, if you are stopping just short of failure and if you are lifting light weights, there's room for even more intensity.  You are not lifting with maximum intensity.  You need to rethink your workouts and up the intensity even more.  Go until you cannot go anymore and then push yourself just a little bit further than that.  Most people will stop short of maximum intensity and that's why I always say to go as far as you think you can go and then push further.  It will get you through that mental block.

Are you working out according to the plan you created?

You plan to workout Monday, Wednesday and Friday.  Are you waking up on Monday and deciding that this week Tuesday, Thursday and Saturday will be better?  Are you moving around the time of your workouts for no good reason?  Consistency is about hitting your scheduled workout dates and times.  Your body will get into a rhythm if you stick to a set day and time for lifting.  How will you know if you are truly giving yourself enough recovery time if you are always changing up the schedule?  Stick to your scheduled workouts like they are a requirement and you will see a difference in the intensity of those workouts.  You will start to see changes in how you feel as you begin a workout and how you feel when you end a workout.  It is amazing.

Are you progressing in your workouts?

We are supposed to be able to either increase weight or repetitions each time we hit an exercise on a new workout.  If that stops happening you need to consider that you might be over training.  I know it is hard to accept that lifting 2 days a week might be more beneficial than lifting 3 days a week, but I'm here to tell you that it works if your body needs that extra recovery time. You will know when that happens.  For me, I couldn't get through the heavy lifts (deadlifts and squats) as well.  I'd find myself thinking "Gee, you could've gotten a couple more reps out of that one" and I'd feel bad for it.  That's when I realized I was not getting enough recovery time.  Maximum intensity will do that to you.  You won't recover nearly as fast as you are used to.  It is a completely different world when you are lifting with that kind of intensity.

Use Intensity

If you want to add mass to your body you must lift weights with great intensity.  You don't stop an exercise when it becomes uncomfortable, you stop an exercise when it becomes impossible.  That's the key.  Go to failure and you will use up every ounce of strength your muscles have.  That, in turn, will force growth in your muscles.  The initial results of working with intensity is an increase in strength, but over time that increase in strength will translate into muscle growth.

Cardio

Do your cardio, but do not over do it.  Cardio can be catabolic so be careful with how you apply it to your workouts.  I highly recommend having a metabolic panel done as it will tell you where your maximum "burn zone" is.  You can then get yourself into that zone and be more effective with your cardio.

Forget the Scale

Don't be so worried about your scale weight as it means nothing.  Someone who weighs 200 lbs. but is 20% body fat is not the same as someone who weighs 225 lbs. and is 10% body fat.  The person who weighs 200 lbs. and is 20% body fat only has 160 lbs. of lean body mass (and 40 lbs. of body fat).  The person who weighs 225 lbs. and is 10% body fat has 202.5 lbs. of lean body mass (and 22.5 lbs. of body fat).  Yes, s/he weighs 25 lbs. more, but s/he is in much better physical shape and the body will reflect that.

Eat!

You need to eat if you are going to change your body.  Your body needs to grow in order to be more efficient at burning fat. This means that you need to build a Larger Engine in order to be more efficient at burning fat.  The way to do this is to lift weights with intensity and eat enough to fuel that intensity.  I personally haven't eaten at maintenance for 2 months and I have gained scale weight, but I have lost fat.  If you workout with the proper amount of intensity you will burn fat without the need for cardio.  That is a fact.  The cardio is an added benefit for fat loss, but it is the weight training at the proper intensity that will really make the difference in your physique.