What is the *logic* or *mechanism of action* involved in creating a *time remnant?*

In some scenes, alterations to the timeline would manifest in various different ways. Sometimes people are shown to quickly, and quietly fade out of existence (like **Barry Allen** in the season 2 finale). Other times it can be slow, violent and unpleasant, resulting in CNS failure, shock, seizure, bleeding, etcetera; delayed and allowing course correction (like what happened to **Cisco Ramon**). Other times it causes cataclysmic events on a massive scale (like when **Eddie Thawne** shoots himself in the heart, **Eobard Thawne** disappears and the sky opens up, creating a singularity). Other times a *time remnant* (sometimes more than one) is allowed to be created with little to no further repercussions. 

Furthermore; in the season 2 finale, **Barry Allen** finally goes back to save his mother from being murdered. A time remnant from one relatively earlier point in the same timeline is shown disappearing. Which should prevent the current present version of him (at least) and thus the whole current present situation from existing altogether.

Is there any logic to how this should all apparently be occuring? Or is this just the nature of trying to balance / compromise navigating the impossibilities of science-fiction against science-fact; and turn it into TV entertainment?