This video demonstrates a number of particle effects that went unused in Half-Life 2: Episode Two.
During Episode Two’s development, Valve were working on overhauling the particle system within the Source Engine to replace the previously hard-coded particle system that their previous games had utilised.
One of the changes Valve worked on in addition to this was overhauling the impact effects to use their new particle system. However, unfortunately, this didn’t see the light of day for Team Fortress 2, Portal or Episode Two and instead was disabled by the console variable ‘cl_new_impact_effects’ by default.
In addition to this, the new impact effects themselves were broken in Episode Two’s release, so even enabling the console variable would fail.
As a side-note; Ignore the HUD, level and other oddities in this video, this is my own custom modification that I’ve been using to produce future upcoming videos in relation to the ‘Prison Evolution’ videos. Apologies for any confusion.