In the "better" malloc.c file, the place function handles the splitting of a block of size csize in order to satisfy a request of size asize:

static void place(void *bp, size_t asize)
{
    size_t csize = GET_SIZE(HDRP(bp));

    if ((csize - asize) >= (DSIZE + OVERHEAD)) {
        PUT(HDRP(bp), PACK(asize, 1));
        PUT(FTRP(bp), PACK(asize, 1));
        bp = NEXT_BLKP(bp);
        PUT(HDRP(bp), PACK(csize-asize, 0));
        PUT(FTRP(bp), PACK(csize-asize, 0));
    }
    else {
        PUT(HDRP(bp), PACK(csize, 1));
        PUT(FTRP(bp), PACK(csize, 1));
    }
}