Gameplay Options
Creature Gib - Choose between gibbing creatures into bits, or ragdoll only.
Always Run - Choose between normal HL always run or the sprint system from HL2
Train Fail Safe - If you jump off the train whilst it's moving and this is enabled, brakes will kick in and the train will stop in a shower of sparks.
Houndeye Tinnitus - High pitched whistling sound you get from flashbangs, in this case from the pressure wave of the houndeye.
Auto-aim - assists your aim slightly.
Weapon Fast Switch - Rather than having the HUD selection, then clicking to select the weapon, it switches directly on using the corresponding key - ie 1 for crowbar or mousewheel, but its user defined for keys etc.
Iron-sight - Normally this is a secondary fire feature, you can remove it completely here should you want.
Iron-Sight Sticky - Enabled you press the secondary fire once and it stays in iron sights mode. Disabled and you have to hold secondary fire down to keep iron sights up.
View Bob - Disable to remove weapon movement when walking/running
View Roll - Dictates the amount of tilt the player has when moving side to side strafing. (values 0.0 - 3.5)
Performance Options
DLight Manager - We added this whilst fixing several performance related bugs in levels with very heavy lightmap use and complicated dynamic lighting setups (QE, BP etc). It automatically detects spikes in dynamic lighting that cause performance drops and disables lightmap updating in those areas temporarily to improve framerates.
Muzzleflash DLights - Added additionally after fixing a performance related bug in Questionable Ethics based on the dynamic flash from gunfire on NPCs that caused dlight spikes and drops in framerate in areas with very sharp lightmap scales. A flag was also added to the NPC's to allow for individual dlight disabling in problem areas that affected all systems regardless of specification. In addition a hammer based entity was added to toggle dynamic lightmap updates to counter odd bugs when reloading saves in those areas. (that was a fun bug to fix...)
Battery DLight - Same issues of performance based on the dynamic glow given off by the suit batteries.
Gluon Burn-Trail - To help lower end systems deal with the additional decals added by the trail.
Visual Options
Chromatic Abberation - Creates a distortion of colour around the edges of geometry when taking damage.
http://en.wikipedia.org/wiki/Chromatic_aberration
Noise Shader - Creates a static noise shader effect when near or in radioactive sources.
Hud Post Process - Creates a prettified version of the basic hud, adding in scan lines etc.
Hud Bloom - adds a nice glowy effect to the hud, determined on your preference. (values 0.0 - 6.0)
Most of those are pretty straightforward and some of them are to assist systems that will struggle otherwise.
