Here is what I found, Obviously we do not have Mechanical Pre Tensioner
"Electronic/Pyrotechnic Pre Tensioner
This pre tensioner operates with an ECU and a gas generator system in a similar way to the airbag. In fact the electronic pre tensioner usually works together with the airbag and may even use the same ECU as the airbag and that increases its effectiveness.
An explosive charge is used and causes the gas generator to produce a volume of gas. This volume of gas produces a pressure which acts on a mechanical linkage to pull the seat belt. This pre tensioner is usually located in the B-pillar.
Both pre tensioners are designed to slacken off slightly around the same time the airbag deploys, this can be around 30 milliseconds after the collision. This slight lapse allows the passenger to have a controlled contact with the airbag.
Both pre tensioners are intended to be used once. Once a pre tensioner has been deployed the entire mechanism including the seat belts should be replaced. The seat belt pre tensioner can be found on the front and rear seats to provide all round safety and protection. "
So either the force was not enough for the ECU to say deploy, or it worked and released very fast..
This system is not perfect but is fairly strong. It takes allot to deploy the system because of complaints that they went off to soon, or to easy.