This technique allows you to collect certain gems without them despawning.
Simply put, the game registers that a gem has been picked up, thus your gem count increases, but there is not enough time to remove the gem.
This is accomplished by leaving the screen on the very next frame after touching the gem.
By performing this trick repeatedly, it is possible to acquire plenty of gems.
However, it only works with static gems; this means that the gem needs to be spawned already when you enter the screen, and that it is not connected to a source spawning it.
In combination with requiring precise actions, this highly reduces actual applications in a run.
With that in mind, it makes sense to use this with easily accessible high-value gems like 300 or 1000.
So far, only 2-2 and 4-1 are considered useful.
Here is a video by Zmaster showing setups for both:
Some rocks in this game spawn gems when hit.
They are connected with an actor called GOLD.
Usually, a timer starts to run down upon the first hit, and once the timer expires, the rock will refrain from spawning more gems.
However, with some rocks, you can leave the screen before the timer runs out, and then be able to repeat the process.
There is currently no use of this, as the only rock considered useful does not function this way.