In a collision, our bodies tend to be thrown forward by inertia so the main objective of the seat belt or harness (from now on we will use one term), is to impede this movement and avoid injury.
The functioning of a seatbelt is complex. In this link we are able to see a timeline of the first few milliseconds of a collision, so that we are able to fully understand how a properly fastened and adjusted seatbelt actually functions. In brief, we can say that a close fitting seatbelt opposes the movement of the body, which in a collision, tends to be thrown forward. Once the pressure exerted by the body passes a certain threshold, the seatbelt gives slightly to prevent injury, which otherwise it might cause to the body.
The device that achieves this is called an “overload”, and it has a simple explanation: if the seatbelt were allowed to continue to exert force on the body, without giving at all, it would cause injury to the human body. If it allows a certain (slight) relaxation of tension, the seatbelt will follow the body it is supporting by a few millimeters, allowing the body to decrease it’s escape speed faster while still protecting the occupant. By understanding this principal, we are able to outline the reasons why a seatbelt must be closely fitted before an accident.
Before explaining this, we should mention the seatbelt pretensioners of which there are many types. Their job is to anticipate an accident and tense up the seatbelt around the body, which they do in milliseconds. Cars that are fitted with this device help to minimize possible injuries. If we inhibit the correct function of the pretensioners, the system doesn’t work as it should.
If we imagine for that the seatbelt is only loosely fastened before a collision, we can get a hand between the seatbelt and the body for example, or we have pegged one of the belts to keep it loose, or even that the seatbelt is not working properly, the pretensioners will not work correctly in an accident, it would be too slow to react to the movement of the body. This would be counterproductive.
This can happen if our car is not equipped with built-in pretensioners, the gap (although only millimeters), would allow the body of the occupant to move by this amount before the seatbelt exerts any force. This would result in the harness causing injury and not functioning as it was designed to.
The reasons for tightening the seatbelt before setting off are clear: with the seatbelt tightened the pretensioners (if you have them) will perform properly; if the pretensioner works properly, so will the seatbelt and the airbag will fire at the precise moment to minimize the impact. When using a seatbelt with a child seat, keeping the seatbelt tight (when not using an ISOFIX seat), will be of benefit to the support offered by the CRS. In the same way, make sure that the harness is tightly fitted to the child’s body for better protection.
Adjust the seatbelt properly; tight to the body, it only takes a minute, less time than it takes for the engine to warm up. It is a question of safety, which is the most important thing.