There is some degree of overlap between shuai jiao and chin na.
However, the skills serve a different purpose.

Chin na encompasses a range of skills (misplacing the bones, cavity press, sealing the breath and dividing the muscle). 
Shuai jiao is simply wrestling.

Ultimately, chin na is designed to defeat shuai jiao. 

Pain is the means by which you can deter even the strongest attacker.
The human body responds to pain.
It seeks to get away from pain.

Chin na begins with misplacing the bones; a coarse infliction of pain through joint leverage.
The threat of breakage acts as deterrent, whilst actual breakage renders the limbs useless.
But breaking a limb is extreme and vulgar.

Incapacitation is important, but preventing a person from earning their livelihood seems excessive.
A broken bone may cause long-term problems.

More advanced chin na focus upon short-term intense pain, designed to fill the mind with panic.
When an aversive reaction is promoted, the assailant cannot think clearly.
Such pain is also temporary and fades soon after the chin na ceases.

'Fa jing' means spontaneous energy release or 'jing release'. It is concerned with kinetic energy emission.
It is the means whereby energy can be transmitted from one person to another. 
It is not structure or movement or energy. It is the way in which the energy is emitted.

Fa jing is a means, not an end.

The harder you seek it, the more you try. Trying involves effort, effort causes tension, tension blocks energy and there is no fa jing.

A practitioner learns how to generate an undulation wave.
This is passed throughout your entire structure, storing and releasing kinetic energy.
We develop this wave by learning whole-body movement. Every strike involves every body part moving as one.

The skill cannot be attained by force.
Only when the body has augmented itself with neigong will fa jing emerge without effort.
You need to be soft and relaxed.
You need to let go of your tension.

The fa jing release of energy is akin to a sneeze; the entire body opening and closing in an instant.

In self defence practice, we cannot use fa jing on one another. The outcome is too unpredictable.
We use soft target pads and sticks for fa jing practice.

The gravity component of fa jing is trained separately. We strike one another using gravity in order to develop this skill.

Peter Southwood's main kung fu instructor was Chu King Hung, following the Yang Shou Chung line. 
His other instructors include Peter Newton, Lam Kam Chuen, Yang Jwing-Ming, Bruce Frantzis and Mike Sigman.