A deload is the least popular week in any training program and one of the most productive. The reason it works is that fitness and fatigue accumulate together, and what you can actually do on a given day is the difference between them.
Why it works
Training builds two things at once: fitness, which rises slowly and decays slowly, and fatigue, which rises quickly and decays quickly. Performance is roughly what is left when you subtract the second from the first.
After several hard weeks, fatigue has accumulated enough to mask real fitness. Reduce the load for a week and fatigue falls fast while fitness barely moves, so what emerges on the other side is a genuine performance increase from work you had already done.
This is the same logic as a taper before a race, just at a smaller scale. The full version is in The Shape of the Taper.
How to actually do one
Halve the volume. If you normally do four sets, do two. This is where the fatigue reduction comes from.
Keep the intensity close. Stay near your usual working weights, perhaps ten percent down. Dropping the weight substantially is what makes people feel like they have lost their strength, and it removes the stimulus that maintains neural adaptation.
Keep the frequency. Train the same days. The habit is the thing most at risk during a rest week, and a deload that becomes two weeks off was not a deload.
Do not add cardio to fill the gap. Common, and it defeats the purpose. The point is less total load, not the same load rearranged.
When to take one
Scheduling one every four to eight weeks is a reasonable default and it is a proxy for the thing you actually want to measure. The signals worth acting on:
Performance stalling or falling at the same perceived effort, across more than one session.
Resting heart rate drifting up over a week or more against your own baseline. The most useful objective marker available.
HRV that will not rebound after easy days. A drop after hard work is expected; a failure to recover is not.
Sleep getting worse while fatigue rises, which is counterintuitive and very characteristic.
Motivation falling off. A genuine signal rather than a character flaw, and one of the earliest.
Together those are the picture in the signs of overtraining, and a deload is the intervention that catches it before it becomes a month-long problem.
Who does not need them often
Beginners, mostly. Novices cannot yet generate enough absolute load to accumulate the fatigue that requires a planned reduction, which is why novice programs run for months without one.
The need grows with training age, absolute loads and life stress outside the gym, which is the variable most people leave out of the calculation entirely.
On how many rest days you need between deloads, see how many rest days per week.