The transposer plugin is a great way to transpose your sound in MainStage automatically. There is a difference in the effect depending on where in the channel strip FX chain you apply the transposition. The global patch transpose will transpose your keyboard keys before their notes have anything to do with a channel strip. If you place the Transposer effect as the first slot effect in a channel strip, then you get the same exact result. But if you place the transposer effect after your instances of chord trigger, then you get a slightly different result.
In application, lets suppose you want to apply a -1 transposition and you have a chord trigger with an input trigger note of C#. If you used the global patch transpose feature, then before you even press a key, it is transposing your MainStage keyboard down -1. So if you pressed that C#, a C natural would immediately play, and the trigger note would not be triggered. This is why moving the chord triggers is necessary in the "regular" way of transposing.
But let's suppose you put a -1 transposer effect on the channel strip, but you placed it under the chord trigger instance(s). Now your keyboard midi data first passes through the chord trigger effect(s) without being transposed. So if you press C# then the C# midi data enters the chord trigger effect and triggers the chord trigger. After this, all of the triggered notes pass into the transposer effect, which then transposed all of them -1. So in this way, you are physically playing in the original key C#, but the output is changed to to transposed key C.