Please note, this is a personal opinion and my take on the differences between these two styles of yoga from my study in both. I like and value both practices.
In Restorative Yoga the aim is to be 100% comfortable, cosy and completely relaxed. This is done through using ample props, such as bolsters and blankets. Being this supported and safe-feeling facilitates complete relaxation of the body and mind, encouraging participants to move into deep rest. You might be in complete silence throughout the session, with poses held for 5-20 minutes. Some teachers might prefer to guide their students through meditation, breathing or mindfulness techniques. It's blissful, slow, soft and nourishing practice, targeting and soothing the nervous system.
Yin Yoga is a strong stretch practice + is definitely not 100% comfortable!
The poses may be held from 1-5 minutes. In a pose it’s encouraged to let go of muscular involvement to target the deeper connective tissues of the body such as fascia
Some schools of Yin will teach Yin as a ‘cold’ practice, done without a warm-up as a way to protect students from going in to the held stretches too deeply
In poses, you might be guided through mindfulness techniques to draw your awareness to letting your muscles relax, to target your ligaments, fascia and joints instead
Yin might be considered a useful practice for building bone density
Yin is a form of resilience-building, as holding the poses can be a uncomfortable and challenging
Yin Yoga focuses on the meridian channels of the body, believed in TCM to boost organ health and overall general wellbeing
MY PERSONAL OPINION
When teaching Yin, I don't recommend holding poses at the edge of your comfort zone. Done 'properly', you're encouraged to disengage your muscles as much as you can, 'hanging out' using your ligaments, tendons and fascia. For that reason, I personally would not recommend Yin to the hypermobile or within 6 months of giving birth.
Done mindfully, Yin is a superb addition to your yoga practice, complementing a Yang (warm, active) practice beautifully. It’s all about balance