Make a func_brush (covered in the playerclip texture), give it a name,
"field1" in this case, then set the solid BSP to "yes", then in
solidity area set it to toggle. This will be your blocker brush (that
you can enable/disabloe the player to pass through) Have a trigger have
an output set to "ontrigger" (for intance), the have it target the
func_brush "field1" in this case, and have the "via this input" set to
disable (this will allow the player to pass through it) and set the
"via this input" to enable to make it so the player CANT pass through
it. Thats what stops the player from moving through it.
Now for the thing that makes it fade away you use the texture called combineshield thats
all (the texture automatically is set to fade away), you only need to
turn that into a func_brush and set its solidity to none so the player
isnt blocked by it.
There are 2 models that are used to simulate the shield generators,
they are models/props_combine/combine_fence01b.mdl and
models/props_combine/combine_fence01a.mdl
By default their skin is set to on, but you can change the way the
generators look (so they look as if they are turned off) by setting the
skin to 1 instead of 0. Make them prop_dynamics and give them a name.
"shieldgen" in this case, have an entity trigger it and have the output
on the triggering entity set to "on trigger" target the props
"shieldgen" in this case, and have the "via this input" set to Skin
with a parameter of 1 (for off) or 0 (for on).
There is a prefab called combine_shieldwall, look at it for everything and more.
Posting And You