To clearify what arletiss said a bit: All you have to do is leave out the collision in the .qc. So if you have a QC like this:
$cd "E:\Users\Jasper\Modeling\props\chair reference"
$modelname "props_c17/chair02a.mdl"
$model "Body" "chair02a_reference.smd"
$cdmaterials "models\props_c17/"
$hboxset "default"
$hbox 0 "static_prop" 0.000 -5.955 -14.632 27.942 15.979 25.172
// Model uses material "chairchrome01.vmt"
// Model uses material "chairfab01.vmt"
$surfaceprop "metal"
$keyvalues { prop_data { "base" "Plastic.Medium" } }
$illumposition 13.971 5.012 5.270
$sequence idle "idle" fps 30.00
$collisionmodel "phymodel.smd" {
$concave
$mass 20.0
$inertia 1.00
$damping 0.00
$rotdamping 0.00
}
To make it an effect you leave out the collision:
$cd "E:\Users\Jasper\Modeling\props\chair reference"
$modelname "props_c17/chair02a.mdl"
$model "Body" "chair02a_reference.smd"
$cdmaterials "models\props_c17/"
$hboxset "default"
$hbox 0 "static_prop" 0.000 -5.955 -14.632 27.942 15.979 25.172
// Model uses material "chairchrome01.vmt"
// Model uses material "chairfab01.vmt"
$surfaceprop "metal"
$keyvalues { prop_data { "base" "Plastic.Medium" } }
$illumposition 13.971 5.012 5.270
$sequence idle "idle" fps 30.00