_sass/vendor/bourbon/css3/_transition.scss

// Shorthand mixin. Supports multiple parentheses-deliminated values for each variable.
// Example: @include transition (all 2s ease-in-out);
//          @include transition (opacity 1s ease-in 2s, width 2s ease-out);
//          @include transition-property (transform, opacity);

@mixin transition ($properties...) {
  // Fix for vendor-prefix transform property
  $needs-prefixes: false;
  $webkit: ();
  $moz: ();
  $spec: ();

  // Create lists for vendor-prefixed transform
  @each $list in $properties {
    @if nth($list, 1) == "transform" {
      $needs-prefixes: true;
      $list1: -webkit-transform;
      $list2: -moz-transform;
      $list3: ();

      @each $var in $list {
        $list3: join($list3, $var);

        @if $var != "transform" {
          $list1: join($list1, $var);
          $list2: join($list2, $var);
        }
      }

      $webkit: append($webkit, $list1);
         $moz: append($moz,    $list2);
        $spec: append($spec,   $list3);
    }

    // Create lists for non-prefixed transition properties
    @else {
      $webkit:  append($webkit, $list, comma);
      $moz:     append($moz,    $list, comma);
      $spec:    append($spec,   $list, comma);
    }
  }

  @if $needs-prefixes {
    -webkit-transition: $webkit;
       -moz-transition: $moz;
            transition: $spec;
  }
  @else {
    @if length($properties) >= 1 {
      @include prefixer(transition, $properties, webkit moz spec);
    }

    @else {
      $properties: all 0.15s ease-out 0s;
      @include prefixer(transition, $properties, webkit moz spec);
    }
  }
}

@mixin transition-property ($properties...) {
   -webkit-transition-property: transition-property-names($properties, 'webkit');
      -moz-transition-property: transition-property-names($properties, 'moz');
           transition-property: transition-property-names($properties, false);
}

@mixin transition-duration ($times...) {
  @include prefixer(transition-duration, $times, webkit moz spec);
}

@mixin transition-timing-function ($motions...) {
// ease | linear | ease-in | ease-out | ease-in-out | cubic-bezier()
  @include prefixer(transition-timing-function, $motions, webkit moz spec);
}

@mixin transition-delay ($times...) {
  @include prefixer(transition-delay, $times, webkit moz spec);
}