  We highlight four points which have been ignored or underestimated before and
which allow a better understanding of "Sur la dynamique de l'electron": (i) the
use by Poincare of active Lorentz transformations (boosts); (ii) the necessity,
required by mechanics, of a group condition l=1 eliminating dilations; (iii)
the key role of the action (electromagnetic or not) and of its invariance; (iv)
the mathematical status of electron models as example or counter-example.
