Hi to adjust the boost level you tee into the line going into the wastegate actuater diaphram, and using a boost controller( just a fancy valvewith fine adjustment really) you are introducing a metered leak in this circuit.
The result is that the wastegate wont open untill the turbo is creating more pressure than the diaphram spring needs because its not getting full manifold pressure . the down side of draw through is
that the vapours that the boost controller bleeds off are flamable as said in the threads attached to the pics posted earlyer in this thread , hence looping it back into the air cleaner.
On the topic of boost controlers Ive known cowboys who have used bbq valves as boost controlers.