¶ Introduction
Super Physics Deformation (Deform), or SPD for short, is a glitch produced by a stuck func_tracktrain interaction, which causes objects that directly blocked it to be able to launch the player at huge speed in a seemingly random direction, when stepped on. It is generally not useful due to the required setup for the object, but can be in rare scenarios.
An example of using SPD launch in Episode Two:
https://www.youtube.com/watch?v=xr6k--HihVA
¶ History
This glitch was first observed at least as early as October 14, 2007 (4 days after Episode Two release), and it was then posted by DemonStrate on HL2DQ forum in the thread titled "Episode 2 Glitch…" (re-hosted on SourceRuns forums). It was the infamous elevator's pickaxe that started it all.
The origin of the name itself is simple, the physics being deformed part was already used to describe stuckjumping around the time when Half-Life 2 Done Quick came out (March 8, 2006), but this time it's essentially a super version of that trick.
From what used to be HL2DQ description on Speed Demos Archive for stuckjumping:
<...> and deform both the physics of both objects. If the animated object isn't necessarily an object, such as a door, then only the regular object's physics will be deformed. When an object's physics are "deformed", it is holding large amounts of energy trying to push itself out, which can be transferred to the player when you touch it. <...>
One can say the glitch was somewhat popularized by DeSinc along with other tricks in their series of Half-Life related videos.
On March 26, 2024, tmob found that the glitch is likely caused by the dmg keyvalue of the train.
The technical explanation of the trick wasn't really concluded up until June 22, 2026.
¶ TODO: to edit everything below to reflect current knowledge
¶ Performing
To activate SPD, all that is needed is for an object to touch a stuck func_tracktrain, that has a high enough dmg keyvalue. It doesn't matter if the object itself was stuck, just the train or its children (in which case, only the stuck child would produce SPD).
If everything is set up correctly, stepping on or off the affected object will launch the player at a certain speed in a somewhat random direction, provided it has collision with the player. Although this can be bypassed by approaching the affected object from something else on the same height, allowing the player to stand on it freely until trying to leave (unless stepping off in the same manner).
The launch direction can be either upward or downward, causing the player to be forcefully thrown onto the ground.
Trains with a checked is blockable by players flag generally seem to not produce SPD.
SPD behavior varies depending on the train's direction of movement. Also, different trains override each other's effects on the object, thus those that deal insufficient or zero damage will effectively nulify SPD.
With a high dmg values before launching the player, the prop first "teleports" them, and only then applies the velocity. This behavior may be due to the extremely high speed involved, making the initial boost resemble a teleport, after which the velocity gets capped and can even be in a completely different direction.
This potentially allows passing through triggers in a single tick, thus not activating them.
¶ Examples
There are only several places where an object can achieve SPD. Here is a list of notable func_tracktrains that can cause it (the dmg keyvalue is specified in parentheses).
In 
Half-Life 2:
- d1_canals_01 - both trains (100,000).
- d1_town_02 - an elevator or a tram (10,000).
- d2_coast_07 - a train (100,000).
- d3_citadel_01 - moving Citadel parts (1,000).
- ep1_c17_02 - falling air duct (500).
- ep2_outland_05 - an elevator (100,000).
- se1_highrise03 - both elevators (999). These trains seem to have
is blockable by playerflag checked, and yet they are still blockable. Not only that, but whenever they are stuck, any prop looses collision with them and falls through, which also activates SPD.
In 
Dark Messiah none of the trains seem to have the requirements for the glitch to work as efficiently as in Half-Life 2, but it's still technically possible — though almost unnoticeable (150 at most).
In 
Portal all func_tracktrains that are capable of SPD have 0 or 15 damage assigned to them, which is not enough to even notice.
¶ Technical explanation
The speed at which the prop launches the player is directly proportional to the dmg keyvalue of the func_tracktrain entity, maintaining a 1:1 ratio. However, once the interaction stops, the velocity gets capped at 3500 ups in any of the three directions (or all directions simultaneously). The speed applied by low damage values is so insignificant that it can easily go unnoticed. It is possible to observe the actual speed by boarding any transport affected by SPD or by enabling god mode and getting stuck, effectively blocking the train's path.
Properties of the train in d1_canals_01. Damage on Crush (dmg) keyvalue is set to 100,000.In addition to dealing damage, the train applies velocity to the entity to push it out of the way. However, unlike players, the prop does not lose it and instead, essentially, inherits.
Lots of things are still unclear. It is unknown why only stepping on a prop causes it to launch, and why exactly objects inherit this push logic. The glitch itself wasn't really looked into that much, remaining a funny niche thing.