twbs--bootstrap/scss/_transitions.scss

.fade {
  opacity: 0;
  @include transition($transition-fade);

  &.show {
    opacity: 1;
  }
}

.collapse {
  display: none;
  &.show {
    display: block;
  }
}

tr {
  &.collapse.show {
    display: table-row;
  }
}

tbody {
  &.collapse.show {
    display: table-row-group;
  }
}

.collapsing {
  position: relative;
  height: 0;
  overflow: hidden;
  @include transition($transition-collapse);
}