From Quake Wiki

.float basebone;
.float baseframe;

This extends ssqc so that it may define a second set of animations on a skeletal entity. All bones up to and including the base bone are subject to the base animation selected according to the baseframe field, and will ignore the animation controlled by the regular frame field.

As this controls the lower-numbered bones, and the upper-numbered bones inherit their positions from lower-numbered bones, the base animation is expected to be used for the legs while the regular animation is used for the torso.

If csqc is supported, then FTE_CSQC_BASEFRAME must also be supported.