Good inputs from 2LP and hac.
My concern is that this historical call is now misused or misrepresented in modern aircraft. In olden days as aircraft were cleared for take off in lower visibilities there was increasing need to use instruments to check that a climb had been established before retracting the gear. At first the altimeter was used due to lags in the old steam driven VSIs. With the advent of IVSI then this instrument was used. In modern aircraft, the VSI may have greater inertial input or even be totally inertial; the result of the improved display is that the positive rate is established more accurately, but also more quickly with the danger of rushed calls and procedures.
Nowadays the crew have lost the big picture: for most take-offs the positive rate can still be established from external visual cues. The instrument check is a good and necessary monitor for the difficulties in larger aircraft or the avoidance visual illusions, but it tends to keep the monitoring pilot head down, whereas after lift off some of the monitoring task is head up and less dependant on instruments. Using the call in a CRM way could be a trigger for the change in monitoring technique.
Also, back to basics as to why the crew require a quick gear retraction and hence a quick check that the aircraft is climbing. The situation would be following an engine failure before V2; it is critical that the positive rate call is accurate; hence use instruments, but how many monitoring pilots are aware of the range of climb rates (often very low) after an engine failure? Room for error; so teach the big picture, positive rate is VSI, and altimeter, and aircraft system, that will subsequently include gear up. No need to rush the normal operation, but the failure case has to be correct every time.