INITIAL_MOMENTUM can be used to apply an impulse load as follows:

Identify a 8-noded brick element, or a group of brick elements, where the
impulse load is to be applied. 

Compute the momentum of the impulse load as integral of the force over time:

https://en.wikipedia.org/wiki/Impulse_(physics)

As force is a vector, the impulse will also be a vector. Specify this vector
quantity as MX, MY, MZ in the card. The deposition time is the time at which
the impulse is applied.

Internally, LS-DYNA will distribute the momentum to the nodes of the element,
compute the corresponding velocity at those nodes, and add that to the current
velocity.