This is quite a technical sales rhetoric, but the concept is really straight forward. What you do here is firstly offer a 3 thirds retained option, focusing the benefits around the exclusive nature of retained search and then offer a contingent exclusive option – which will sound very similar to the retained option, but with no upfront fee. Having done so you then ask the client which of these options they prefer.
Often the proportion of contingent exclusive work increases significantly when positioned this way. The psychology is that rather than the client feeling the contingent exclusive is the ‘upsell’ of contingent non-exclusive, they feel that it is the ‘softer’ option against retained. You will even end up with the odd retained role!