boost controllers

i've seen these boost controllers and ecu piggyback for turbo/supercharged cars like apexi, greddy and blitz that somehow control the boost and the air/fuel ratio. but i dont know how they controll the boost, are they somehow connected to the wastegate? and i've see some that have programable maps, what does this mean? what are the benefits/downsides between manual and automatic controllers? i'm really confused and new to this so if u could just bare w/ me, i'd appreciate it.

Every turbochargers boost is controlled in the same way (except for VNT's), boost pressure (usually from the manifold), is sent through a signal line, where it reaches the wastegate actuator and overcomes the spring load to open the wastegate. So if your actuator spring load is 5 pounds, it will take 5 pounds of boost in order to open the wastegate. In order to run higher boost pressure, a boost controller will create a controlled leak in the signal line. This way, the wastegate only ever sees the regular 5 psi, regardless of boost pressure. So if your wastegates spring load is 5 pounds, and your boost controller was set at 7 psi, your boost controller would bleed off 2 psi from the signal line. This doesn't change the amount of pressure it takes to open the wastegate, but instead sends 5 psi to the wastegate instead of the 7 that is in your manifold. So if you wanted to run 14 psi, and the wastegate opened at 5, the boost controller would bleed off 9 psi before it reaches the wastegate. Basically, boost controllers work by hiding the extra boost from your wastegate. Manual and electronic boost controllers work the same way, by bleeding off boost in the signal line. Electronic boost controllers are generally better, however, as they are more accurate, and offer a variety of extra functions. Many electronic boost controllers will block the signal line completely, until it reaches the set pressure to open the wastegate. This is because, although the wastegate may open completely at 5 psi, it might open a fraction significantly before it reaches 5 psi, and as a result, your turbo will build boost slower. So a good wastegate can actually help your turbo spool quicker.

As for maps, that has to do with fuel management, not boost control. Piggyback FMU's work by intercepting the fuel signal after the CPU sends it, and modifies it to add more fuel.