NFHS 7-4-4 fumble = touchback

[Jim S]

Wrong. The determination is force.
B(R) does not have to have possession in order to apply a new force to the ball.
K kicks to R. R runs to the R10 and fumbles, K picks up the ball and runs to the 5 where K fumbles. The ball comes to rest at the 2 and R in attempting to retrieve the ball accidently kicks it into the EZ and across the endline. Result is a safety. R's force is what put the ball into the EZ.
DO NOT get trapped into this on a kick though. A kick ends when the ball becomes dead by rule, or a player gains possession. This may be what you are thinking of.