This is typically set up with a frozen wall-crawler, but can include other frozen enemies or Snails, Trippers, etc.
The enemy can be up to 3 pixels higher than the adjacent runway and still have benefits.
At 3 pixels above the runway, Samus can run off of the enemy and not lose speed, which is often useful for cross-room speedy jumps and jumps into water.
At 2 pixels above, single-stutter shortcharges often fail and multi-stutters are unreliable.
At 1 pixel above, multi-stutters are fairly reliable.
A pixel-perfect positioning is necessary when using an enemy to bridge a gap, where Samus needs to run onto and off of the frozen enemy.
If the enemy is even 1 pixel lower than the runway, it cannot be used.
See also: Tech Page