Instinctive training basically means that training intensity should be based on how you feel during a given session. In essence, you use a combination of experience and "instinct" to guide your day-to-day workouts. This helps address the fact that outside influences such as sleep patterns, nutrition, stress, and aches and pains can significantly affect your ability to train at peak levels on a daily basis.
Emerging research seems to support an instinctive training approach. In scientific circles, this strategy has been termed "autoregulatory training" and researchers have evaluated its applicability in various flexible periodization models.
In one study (McNamara et al., 2010), sixteen beginner weight-training subjects were divided into two resistance training groups. All the participants performed the same volume of training and the same rep max schemes (10RM, 15RM, and 20RM) twice a week, except one group performed the routine in a regimented fashion while the other group was allowed to choose when to perform the given workouts based on how they felt.
At the end of the 12-week training period, those who were allowed to train instinctively increased leg press strength by 62 kg compared to only a 16 kg increase in the control group.
Best Suited for Experienced Lifters
Now, despite these impressive results on novice subjects, instinctive training is better suited to more advanced lifters. You must be highly in tune with your mental, physical, and emotional state and how these factors affect training capacity. This only comes with dedicated training experience. A beginner simply isn't skilled enough in the nuances of training to accurately gauge his daily capacity for peak performance, which can lead to substandard results.
It's also important to understand that instinctive training has a downside. Namely, unless you're highly motivated to train, it becomes tempting to give in to laziness. The flexible nature of the strategy provides a ready-made excuse to take things easy, even if you're feeling good. Be honest with yourself. If you're the type that needs regimentation to keep you on the straight and narrow, then a more traditional periodization program is a better option.
On the other hand, you can't take this to the other extreme and think it's okay to push through anything short of a broken bone. Remember that the body needs adequate recuperation to regenerate its resources. Otherwise progress will ultimately hit a wall. So regardless of subjective feelings, make sure to include regular unloading cycles throughout your training program.