Momentum and impulse
Momentum is mass times velocity. Spreading a collision over more time lowers the force.
A worked example
A 60 kg passenger stopping from 20 m/s in 0.20 s feels 6 000 N. An airbag stretching that to 0.60 s drops it to 2 000 N.
The mistake people actually make
The change in momentum is fixed by mass and speed. Crumple zones change the TIME, not the momentum.