From Quake Wiki

Field definitions:

.void(vector vImpactVelocity) movetypesteplandevent;

This field function, when provided, is triggered on a MOVETYPE_STEP entity when it experiences "land event".

The standard engine behavior for this event is to play the sv_sound_land CVar sound.

The parameter provides the velocity of the entity at the time of the impact. The z value may therefore be used to calculate how "hard" the entity struck the surface.

NOTE: If this field function is provided on a MOVETYPE_STEP entity, the standard sv_sound_land sound IS SUPPRESSED to allow for authors to create their own feedback.