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A Big Picture EventStorming typically involves many first-time participants who need to be guided along the many steps of the workshop.
A big explanation upfront can backfire spectacularly. Explaining everything you plan to do before doing it will increase participants' cognitive load. Providing too much information before a hands-on experience can only lead to a spiral of questions. See also Do first, explain later.
The same principle applies to the Visible legend ; detailed information about the following can trigger questions that are disconnected from the moment's focus. Or can invite some participants to anticipate the following steps, confusing the others.
Embrace the idea of providing just enough guidance to complete the current step while keeping participants' cognitive load under control.
Use a Visible Legend as a placeholder for the official notation, but make it clear that more elements of the notation will be defined step by step.
Begin with the bare minimum: Events and their verbal form. Then progressively add new elements of notation the moment they become necessary. Special Events, such as time-triggered events or interrupt events , are usually introduced when someone asks for help on a corner case.
This moment usually signals that the current notation has been digested, and the participants are ready for something more sophisticated. When this happens, add the new notation to the legend and provide a short explanation.
After completing a specific step, you may ask participants questions like "What is the aspect you'd like to see in the model?" or "What is the next thing you'd like to see?" The answers are usually not very different from the default next step, but your facilitation may benefit from the interaction: now the whole crew is in charge of it.
Sometimes asking the audience what they'd like to see next can lead you in interesting, unexpected directions, so make sure you have some extra colours in your bag.
The Incremental Notation approach is a way to provide some training wheels to a modelling team that is not familiar with the grammar. In Big Picture EventStorming, this is almost always the case, so incremental notation is strongly recommended there.
Process Modelling and Software Design are supposed to happen more frequently, and with a more stable team, so we dropped the incremental notation and embraced a collaborative games approach for these formats.
You can still use Incremental Notation there, as a tactical choice. But you'll probably move faster by dropping the training wheels.