What is the centre line?
The centre line refers to the vertical axis dividing your body in half or your opponents body in half. This line extends forward from your body. The majority of Wing Chun techniques originates and travels along the centre line. The centre line is the quickest route between you and your opponent or straightest line between A to B.
What is the fixed elbow position?
The fixed elbow position refers to the Wing Chun theory of always keeping the elbow a fist away from the body and as close to the centre line as possible. If the elbow maintains this position (with the rear hand always near by) all defensive and attaking techniques will be easier to perform and the body well guarded. Note the elbow does not always have to be exactly in the centre as long as it is not against the body, and the forearm is in the centre. (The above idea applies specifically to the front hand position.)
What is Pak Sau?
This is a drill some times called slapping block. The drill is performed with a partner and is used to instil a sense of timing. And to develop attacking and counter attacking skills.
WSL Wing Chun Terminology courtesy of David Peterson
Siu Nim Tau - (First Form) "Young Idea"
Cham Kiu - (Second Form) "Seeking/Finding The Bridge"
Biu Ji - (Third Form) "Pointing Fingers"
Muk Yan Jong - "Wooden Dummy" Form
Luk Dim Boon Gwan - "6.5 Point Pole" Form
Baat Jaam Do - "Eight Slashing/Cutting Knives" Form
Yi Ji Kim Yeung Ma - "Character Two Goat Gripping (Basic) Stance"
Saam Gok Ma - "Triangular Advancing/Retreating Stance"
Saam Gok Bo - "Triangular Advancing/Retreating Footwork"
Dui Gok Ma - "Side-facing Stance"
Juen Ma - "Pivoting/Turning Footwork"
Seung Ma - "Forward Attacking Footwork"
Tui Ma - "Defensive (Side-stepping) Footwork"
Yat Ji Kuen - "Sun-character (Basic Vertical) Fist/Punch"
Taan Sau - "Spreading-hand" Deflection
Fook Sau - "Subduing-hand" Deflection
Bong Sau - "Upper-arm" Deflection
Huen Sau - "Circling-hand" Deflection
Wu Sau - "Guarding-hand" Deflection
Paak Sau - "Slapping-hand" Deflection
Jing Jeung - "Standing (vertical) Palm-strike"
Waang Jeung - "Lying (horizontal) Palm-strike"
Soh Sau - "Pressing Palm" Deflection
Fak Sau - "Whisking/Whipping Hand" Attack
Lan Sau - "Barrier-arm" Deflection
Jam Sau - "Sinking-arm" Deflection
Jat Sau - "Jerking/Dragging/Obstructing-hand" Deflection
Biu Sau - "Thrusting/Spearing-hand" Attack
Naat Sau - "Pressing-down/Extinguishing" Deflection
Daan Sau - "Rebounding-hand" Deflection
Gaan Sau - "Splitting/Dividing-hand" Deflection
Che Jeung - "Slanting/Descending Palm-strike"
Dai Jeung - "Underneath/Lifting-palm" Deflection
Senk Sau - "Scraping/Shaving-hand" Deflection
Dang Geuk - "Ascending (Basic Heel) Kick"
Waang Geuk - "Horizontal (Side) Kick"
Chi Sau - "Sticky-hands" Exercise
Chi Geuk - "Sticky-legs" Exercise
Luk Sau/Poon Sau - "Rolling-hands" Exercise
Daan (Chi) Sau - "Single-hand" Chi Sau Exercise
Seung (Chi) Sau - "Double-hand" Chi Sau Exercise
Gwoh Sau - "Free-attack" Chi Sau Exercise
Laap Sau - "Warding-off/Deflecting-hands" Exercise
Lat Sau Jik Chung - "Constant Forward Springy-energy" Concept
Chiu Ying - "Square-on/Facing" Concept
Jui Ying - "Chasing" Concept
Wing Chun is not about this technique, that stance or those combinations;
it's about applying logical concepts in a totally adaptable way to whatever situation confronts us.