Normally UGens are defined within the scope of a shred, this may be a sporked function or the "main shred", being the code in a file outside of any functions or classes.

When the shred a UGen was defined in exits the UGen will be disconnected from anything it connected to (I'm not sure everything that connects to the Ugen on the other end will also be disconected but that doesn't matter much here). Because of this the DAC will no longer poll it for new samples so it won't take any CPU anymore. This, in adition to calculation order, is the big advantage of the "pull through" model we use for the UGens.

Kas.