If the call is "Mirror Chain Reaction", you would definitely pass left-shoulders.

On calls such as "Left Pass Thru", "Left Pass & Roll", and "Left Travel Thru", you would also pass left-shoulders.

In many cases, "Left" means to do the mirror image of the call. However...

On "Left Spin The Windmill In", the "Left" applies only to the centers. The Ends do not 1/4 In then pass two dancers "Left" shoulders.

On "Interlocked Left Scoot & Plenty", the stars are danced by passing right-shoulders (a normal Reverse The Pass).

In the first case (Left Spin The Windmill In), there are actually two directions given to the dancers: 'left' applies to the centers, and 'in' applies to the outsides. If no direction is given to the centers, they assume 'right'.

In the second case (Interlocked Left Scoot & Plenty), the word "left" is used as a helper word. It assists the dancers in using the left-hand for a scoot back, even though they should know darn well to use the left-hand! On the floor, it is often the case that dancers on are auto-pilot, or the outsides may not be paying attention to the handedness of the center wave. The word "left" is simply a helper word.

Similarly, at Advanced and below, the word "left" is often used as a helper word in order to increase dancer success (e.g., "left scoot back", "left switch the wave", etc.). So, the question becomes... is the word "left", in the phrase "left chain reaction", a helper word? If so, the pass thru is a right-shoulder pass, if not, the pass thru is a left-shoulder pass.

By the way, I generally try to avoid the issue (i.e., avoid saying things such as "left chain reaction") by using other words. For example:

1. "It's a lefty, chain reaction"; or
2. "Centers left swing thru, all chain reaction" (I use one or more left-hand calls to the centers prior to calling chain reaction).