In Airbags with reference geometry (*AIRBAG_REFERENCE_GEOMETRY), when the airbag bag element in folded state is smaller or larger than the reference state, stress needs to develop. LS-DYNA suppresses these initial strains so no stresses develop to avoid bag distortion and to improve stability. This may cause the bag to deploy but its final shape may not be accurate when compared with the intended bag shape.
If you induce the strains quickly, then the bag geometry is altered during the early simulation cycles. To avoid this, it is necessary to induce the strains gradually ie over a certain period of time. That is where the TSRFAC with a load-curve comes into play. You simply define a curve whose x-axis is time and y-axis is a scale factor for the strain.
The choice of time is user-dependent but if you can induce the strains before the bag attains fully deployed state,
the bag can potentially deploy whose final shape may be much closer to the desired shape.
For more information, please refer to the Remark section in the LS-DYNA Keyword Users Manual.