Movement Tubes
Works well, nice work.
Dunno if it was intentional or not, but the player moving through the tubes goes faster than the energy balls, so if you put a ball in there, then jump in after it, you zoom up and hit it and die.
completely intentional 
Just tried this and it didn't work. Can you be more specific?
That is probably a pretty terrible way of doing it, I'd suggest going for the other methods.
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I believe the command is actually "+duck". If you use this method, you must also use the command "-duck" when you leave the tube, or the player will be stuck permanently crouched.
No, sadly no cake for mappers who force players towards an action, even if its just crouching, there is no reason to force the player into a crouch, unless hes blind, then he shouldnt even be playing at all.
Tell that to Valve. They used a point_clientcommand to force the player to crouch on their movement tubes.
I'm all against forcing players to do things but if you don't force them to crouch they complain about "I got stuck and had to crouch"
IT HAPPENED TO ME - I sent a map out to be tested and thats exactly what they said. so forcing them to crouch is a must
Yeah I can't see any other way of doing these tubes in my map without forcing the player to crouch, because the player does not enter them intentionally. Perhaps if, like in regular portal, you get in the tubes voluntarily, then it makes sense for the player to crouch before they enter. But not in my map :p