Human body is fantastic; with good training planning, it's able to overcome a level of effort that it was previously impossible for it to do. This proces is called 'Adaptation'.

This fitness change is not an inmediate improvement but it takes time to adapt to a certain training load.
The key factors of the adaptation are in fact three factors : Training Load, Fatigue and Recovery.

During a training session, riders begin to experience fatigue at the end of this session, recovery process begins. Body repairs the little damages caused by the workout and will prepair it to be able to do a little higher intensity workout. This phenomenon is called overcompensation.

Playing with this phenomenon is fantastic for rider & Coach.
If the training session are well planned and structured you can benefit a serial of overcompensation processes and rider fitness will increase.

On the other side, if training load is too high or recovery time is insufficient; the fatigue will increase dramatically and fitness will fall.
That's what we call overtraining or burn-out. To avoid to be overtrained; Riders & Coach have to build together a high level of communication based on training tools data and rider feedback.
If a rider is overtrained, the only remedy is resting and Active Recovery rides (after checked Rider Health Status).

 An other case which is quite common is allowing too many recovery between training session. That's for example the case of occasional riders who have only the week-end to ride and could not take advantage of the overcompensation phenomenon.

If you just have the week end to ride; try to have additional session at mid-week even if it's not a bike session. Following this way; your fitness could slowly increase and you will not loose all your week-end training effects.

As you can see, the training process is quite complex as every rider is different and every time to overcompensate will differ according of the type of training (strength, steady state LT itntervals, resistance work, ....); building a good training plan requires experience and lot of communication between trainers & trainees.