Stopping time and crash force
The same change in momentum spread over a longer stopping time means a smaller average force on the body.
A worked example
A 50 kg passenger stopped from 20 m/s in 0.40 s feels (50 x 20) / 0.40 = 2 500 N, and over 1.0 s that falls to 1 000 N.
The mistake people actually make
A belt or airbag does not reduce your change in momentum, it only stretches the time over which it happens. Reaction distance is covered at full speed before the brakes are touched at all.