Simulation of disabled user characteristics can be useful in two cases: (1) during preliminary tests of interactive systems, (2) to train designers and make them understand the difficulties encountered by these users with special needs. This paper describes a case study involving a preliminary user test of a system called ComMob (Communication and Mobility). It is a communication aid for people with motor disabilities of the dyskinetic Cerebral Palsy type. This software is usable in mobility and may be installed on a wheelchair. A preliminary discount evaluation was carried out in the laboratory with non-disabled participants in whom involuntary movements were induced. These movements were characteristic of users with a dyskinetic Cerebral Palsy disability. The paper focuses on the principles of the discount evaluation that was implemented. The first results are promising and show the feasibility of the approach, leading to numerous research perspectives.