OK, thank you for the answer. I have to check out the vids I've seen where the situation looked like (for me!) illegal, and I 'll see the "front base tackling" thing...
I'll post my results soon
EDIT : I went to youtube, and you 're right, I believe. When big running play happens, the running back hits rear or side of the defense player's base everytime. For now I didn't find any vid where a RB hits a D base's front and is allowed to continue his run.
A new rules detail I understand now
I guess details are important, since here in France we doens' t have ANY reference point for MF playing or rules...So I have to be "square" on my rules and play's knowledge, to try to play a game as closest as possible of the one you 're playing in USA/canada (speaking about basics, not advanced rules).