Challenging at the line of the shoulders will be allowed as the tackle height.
This will be reintroduced next month at the elimination stage.
"We need to analyse the data in more detail, but our preliminary analysis has shown 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," said RFU interim chief executive Nigel Melville.
"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 ball carrier and tackler were both upright."
"This was always a trial and we weren't sure what the outcomes would be," added Melville.
"Our two main objectives were to determine whether, through law change, the height of the tackle can actually be reduced and if a reduction in the height of the tackle then leads to a reduction in concussion risk."