You need to use all four straps to secure the wheelchair to the floor of a vehicle! This method is most often used in transit buses, paratransit buses, school buses, and in personally-owned vans. It works well in these settings because the straps work with many types and styles of wheelchairs. Some bus drivers, transit aides, or passengers might want to use only the straps that are easy to reach. However, not using all 4 straps could lead to tipping or too much force on the straps that are attached.
The idea with 4-point securement is to attach the straps to the wheelchair frame just below the combined center of mass of the wheelchair and its occupant. The angles from the floor to the wheelchair are carefully defined within the standard so that they stabilize the chair and prevent tipping, rotation, and forward movement. The standard for testing the strength of the securement of the wheelchair to the floor of the vehicle with straps or a docking system is comparable to the standard used for testing the attachment of a car or bus seat to the frame of the vehicle.
The first video shows the use of all 4 straps. The following show various scenarios such as failing to use securement, using only one front and one rear strap, or attaching the tiedowns to improper wheelchair locations (such as rear wheels).