Prior to 5.1, you use to be able to:

/castsequence Spell 1, Spell 1... (or for that matter, Spell 1, Spell 2, Spell 3... and whatever fall-through you wanted on line 2).
/cast Spell 2.

If you were spamming at a decent rate (2-5 clicks per second), your Spell 2 would regularly fire off whenever it was available.
It didn't matter if it was: Lightning Bolt, Lightning Bolt and Lava Burst or Flash of Light, Flash of Light and Holy Shock.



This method seems to be very similar to the two-step macros that Mercurio posted for WotLK and were adapted for Cata.
In a single macro, you have two steps...
The first line tries to fire first, but has an effect (the 1) that does nothing.
Which allows the second line to fire off, (the Lava Burst or Holy Shock).
At which point the second press gets the spell effect from line 1.

Essentially, if you spam quickly enough, you will get your fall-through or proc ability on cooldown virtually every time.
With half of your keystrokes attempting to execute the main spell, meaning with a fast click rate that should go off on GCD anytime the fall-through is not.

You could accomplish the same with a priority sequence that sends: 1, 2, 3, and 4.
With abilities on each of those keys.
If you mash quickly enough, in theory one of those spells will be available for casting.
This method (above) is like a priority, with only two keys instead of multiple... with the benefit of being a single macro rather than multiple abilities.

It (unlike a priority) is a work around, allowing fall-through spells to fire off despite Blizzard recently (this patch) removing that ability.
As such, using it is an exploit.
But one that is unlikely to cause a suspension/ban, because you're using their in-game system.
The same as Blizzard breaking addons like oQueue and AV Preform Enabler, but likely not stopping people from coordinated queuing in a vent channel.
If you want the same effect, without exploiting the macro system (which was changed this patch to remove this functionality), use a priority system with the popsicle method.