It was the correct way (the transition Non was followed only once in fact). I've made a coding error later that made my trace be triggered twice.

So the correct way of doing a Decision in Java code is to code directly in the Source tab of your processdefinition.xml the following code :

<decision name="Client actif ?">
<handler class="fr.jl2tho.tutorialjbpm.action.IsCustomer" />
 <transition name="Oui" to="Enregistrer la Question"></transition>
 <transition name="Non" to="Prevenir le commercial"></transition>
 </decision>

Where IsCustomer implements org.jbpm.graph.node.DecisionHandler.

It seems to me impossible to do it through the GUI ProcessDesigner.