When you bind a fixture to an instance using physics_fixture_bind this returns an "id" for the bound fixture. You can use this id to set the restitution value of the bound fixture, not the "base" fixture, at any time using this function. Restitution is usually set as a value between 0 and 1, but you can use higher values if required, although the results may be unpredictable.
|fixture||the id of the bound fixture|
|restitution||the new restitution value to apply|
var rest = physics_get_restitution(fix_id);
physics_set_restitution(fix_id, rest * 2);
The code above gets the current restitution value for the bound physics properties of the instance and then sets them to a different value.