Subframe is a general name for technology that abuses particle update order.
Every frame in The Powder Toy all the particles are updated one after another according to their ID.
When you hover over a particle the ID is visible next to X and Y values but only when the debug HUD (toggle it with "D" key) is enabled.
Every time you load a save all the particles are ordered from left to right and top to bottom. That means they will also be updated in that order. You should always assume that order or your contraptions can break when reloading the save.
How to build it: A solid spark is made using at least 3 particles:
Their IDs have to be in that order to work
ID of 1 < ID of 2 < ID of 3
How it works: In the first frame nothing happens and at the end the 1st particle gets sparked by the 3rd particle.
In the second frame:
2nd particle replaces the sparked 1st particle, so its not sparked anymore.
Now that the 1st particle isn't sparked the 3rd particle can spark it again (thus avoiding the spark cooldown and blinking).
So the 1st particle gets sparked, the spark is replaced, and gets sparked again.