The Rolls Royce C20 (or later variants) Maintenance Manual explicitly details the full and correct procedure to use for rinse and/or wash. Sadly none of the 'methods' mentioned below are accurate and/or fully explain it - especially the 'while engine is running' method.
1 of the important details to note are the fluid should be injected for 3 seconds before engaging the starter to prevent the N1 accelerating too much and atomising the water and keep the water injecting for 3 seconds after the engine stops. There are more gems of this nature in the procedure.
Anyway, its all in the MM, have a read, it will make clear and concise sense, follow it and we will all remove yet another 'chinese whispers' screwed-up technique/myth.