@ValeriaBreve
Keep in mind what I mentioned about Excel - PowerApps is just like it! Why do I point this out? Because just like in Excel, if any cell value changes and there are formulas in other cells that reference that cell, they will all re-evaluate. The same thing happens in PowerApps. So, performing calculations based on another control - they will always re-evaluate if there is any change to the other controls they reference. This is one of the awesome features for PowerApps, but very frustrating to those thinking more "development" like.
So, in your case, changing the DisplayMode is not going to cause any particular re-evaluations, unless you have formulas that reference the DisplayMode of the form.
BUT, the underlying value of the Default is going to change what the toggle shows and thus an action that is performed.
And yes, the OnSelect action will only occur if a user clicks on the control. OnCheck, OnUnCheck and OnChange are all different actions. Some controls will perform those actions when values change, others not.
So back to the story - Is this toggle part of a field of your datasource? Or is it just an added toggle?
If just added, then where in the formula-chain is the underlying datasource column value?
Does LabelTotalQuantity.Text come from the datasource column? Or does DataCardValueTransp.Value come from it?