What is active mobilization?
Active mobilization uses the same basic principle as PNF hold relax: lengthened muscles are made to contract isometrically. The difference is that the isometric contraction is not maximal but rather consists of several repetitions of sub-maximal isometric contractions of the target and the surrounding muscles. Read more
What is active stretching?
A fascial network perspective.
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This video illustrates the sequence of actions that are useful for getting more extensibility of the muscle and facial structures in the upper limb. 
Infants born preterm, who are hypermobile, or appear to have low muscle tone, often have limited extensibility in the neck and spine extensors. 
Premature infants often have poor extensibility in the neck extensor muscles, which in turn has an impact on their ability to actively lift the head when pulled to sitting, when being lifted up from supine or when being lowered prone sitting to supine. 
Infants who have joint hypermobility or were born very early often have some tightness in the hip muscles. Gently stretching activities are useful for improving hip flexibility for sitting, crawling and getting up onto hands and knees. 
This video clip illustrates active stretching of restricted muscle and fascial structures that cross over the lateral aspect of the hips. 
This video clip demonstrates how it is possible to mobilize tight hips structures in a very active toddler.