The study found a 24 per cent decrease in all tackles where contact was made above the armpit line and a 25 per cent decrease in tackles by an upright tackler on an upright ball-carrier.
Overall, there was a 41 per cent decrease in tackles making contact with the head or neck of a ball-carrier. However, in comparison with the Championship league competition, there was an increase in the rate of concussive incidents.
"We need to analyse the data in more detail," said RFU interim chief executive Nigel Melville.
"But our preliminary analysis has shown that all of these incidents occurred when a bent-at-the-waist tackler was attempting to tackle a bent-at-the-waist ball-carrier following a short pass from the scrum-half.
"This is an area that the trial was not specifically looking to influence, as the primary focus was to reduce the risk of concussion where the ball-carrier and tackler were both upright. We will be analysing this particular situation in more detail.
"Overall, this has been an extremely valuable exercise. We've learnt a lot and tested an approach to reducing the risk of concussion in a real-life setting.
"We have shown that reducing the tackle height is achievable and we already have useful and detailed data from the first 36 matches in this 43-game cup competition."
Dean Ryan, RFU head of international player development, said: "We knew the areas around the pick and go and pop-off nine were going to be difficult to referee. So, in one area that we said was going to be difficult to referee, and therefore we would apply current law, what we created was an unintended behaviour of somebody bending in a tackle in that situation."
A steering group featured Ryan and RFU medical services director Simon Kemp, as well as analysts and representatives from the RFU's refereeing and medical research departments and World Rugby chief scientist Ross Tucker.
They examined these concussive incidents, considering the possibility of head-on-knee and head-on-hip contact, as well as head clashes between tacklers on the same team.
They accounted for squad rotation and a varying standard of players and tracked potential trends that might result, such as the number of offloads, which did not change dramatically.
Before the trial, in concussive incidents in Championship competition where the tackle occurred above the armpit, 80 per cent - suffered by the tackler - occurred in upright tackles on upright carriers.
During the trial, only 13 per cent of concussions in tackles above the armpit line occurred in such collisions.
However, 63 per cent of the total number of concussions in tackles above the armpit line occurred when both players were bent at the waist - a different pattern of risk to those that had been previously identified by World Rugby's research.
The study group will complete its final report and put it before World Rugby's meeting in France in March.
