There is health in Halo 2 and it is
not simply your shields. Everyone
knows that you can take a few
non-head shots after your shields
go down, but you are not at full
health simply because your shields
come back up. In fact, it may take
up to 6-7 seconds after your shields
come back up before you are at full
health (depending on how much damage
you took after your shields went
down).
As an example of this, take a plasma
pistol charged shot and take down
someone's shields. Then shoot that
player with 5 gutshots using the
magnum (a 6th gutshot will kill).
Now get another charged shot ready
and the moment the player's shields
come back up, hit them with the
charged shot and then you should be
able to kill them with one gutshot
(maybe two if you weren't quick
enough with the charged shot). If
you wait about 6-7 seconds after
shields are up before hitting the
player with the charged shot, it
will again take 6 gutshots to kill
them, so health does replenish and
it doesn't take long after your
shields come back up.
This may explain why it seems that
sometimes in the heat battle when
your shields have just come back,
that you might die quicker than
normal, you were not at full strength
even though your shields were at
full strength.