I guess I need to quote Step 7: entirely then...
SR4, 174, "Step 7: Ongoing Effects
Many spells can be sustained for as long as the magician
is willing to concentrate on the spell, as noted in the spell’s
Duration. While sustained spells do offer the opportunity to
have an ongoing magical effect, they are also draining on the
magician’s magical abilities. For each sustained spell the magician
maintains, she suffers a –2 dice penalty on all other tests.
If an area-effect spell is sustained, the affected area may be
moved with a Complex Action, as long as it remains within line of
sight. Characters who “drop out� of the affected area are no longer
affected by the spell; characters who are “enveloped� by the area
must defend against the effects of the spell as appropriate.
If the gamemaster chooses, certain circumstances may
threaten to break a magician’s concentration while she is sustaining
a spell, such as taking damage, full defense, dropping prone,
and so on. If a magician’s concentration is disrupted while sustaining
a spell, she must make a Willpower + Spellcasting (2)
Test to avoid dropping the sustained spell (note that the sustaining
modifier does not apply to this test)."
You MUST sustain it AFTER the drain.
Also, Street Magic, from the Spell design rules, 160, "If a permanent spell is not maintained for the entire
time required (Drain Value x 2 Combat Turns), it does not become permanent and its effect is lost."
If the effect is lost if not maintained for long enough, then the effect begins before the end of the maintaining.