@Argamis
Wow, you must be really fun at parties.

[THE EXPLANATION]:

||. The Equestria Girls portal (both) work like a Stargate (from the tv series "Stargate"): The person/object must completely cross a "barrier" [the "event horizon"], before any transformation & transportation can happen entirely at once.
 -> The "barrier" is constantly pushing away, so any pasive matter like air, rain/snow, water or sand will be deflected/rejected [while an active living body (or a solid object with enough momentum) will advance through it).
  
. The body initially goes into an "intermediate space" inside the portal (where the passage of time is extremely slow), so any bodypart that enters "first" stops sending or receiving nerve signals from the brain [it does not "feel" like anything, literally (it is not like a "missing" bodypart, but more like extreme sedation)].
 -> Once the head goes inside, the rest of the body either has enough momentum to fully complete the crossing, or it is eventually pulled out by the barrier [this is what happened to human!Pinkie near the end of Rainbow Rocks].
  
. With the body fully inside the "intermediate space" (before even a single heartbeat could happen in the time_dilation), the equivalent of an A.I. performs an analysis to decide how to transform it into one of the current "native species" of "the other side" (that is safe/compatible with its ecology), adding attributes/skills/abilites & even "extras" like magic if necesary. 
-> Then magical processes DESTROYS the body to the subatomic level, sending all of its matter/energy to "the other side", alongside with all the "information" necesary to reconstruct it with the "new specifications" (plus the "original" configuration data to be encripted & stored inside the new body for the future use of any other "portal" that follows apropiate safety protocols).
  
. On "the other side", a similar process happens in reverse [building a "new pony body" atom_by_atom inside a safe "intermediate space" before releasing it].
-> For the traveler it feels like sudden brief loss of conciousness, followed by spatial confusion and quick regaining of awareness (as the new nerves on the hooves re-connect with the spine & the new neurons in the horse_brain re-synchronize).||