Hi Philippe,

Firstly, your English is actually very good, and an awful lot better than my French!  The start of cut finger (the aluminium block our push down) has two purposes.  It both positions the blade in the correct position and by pressing down on it firmly holds the cutting head in position preventing it from moving away from you as the blade enters.  So as you say it is a question of strength when pushing the finger down, you will notice there are teeth on the underside of it to help.

I hope that helps, please let us know if we can be any further assistance.