According to
Heathrow noise: Operational trial
the four triggers are :
Trigger 1: “The anticipated arrival delay or departure delay is 10 minutes or more”
Trigger 2: “The headwind component on approach to Heathrow is forecast to be greater than 20 knots at 3000ft”
Trigger 3: “The arrival or departure flight schedule is anticipated to run later than 30 minutes or 30% of flights are running outside of the 15 minutes punctuality target”
Trigger 4: “Following a period of disruption to recover the operation such as snow”
although the daily reports at heathrowtrial.com show a fair amount of TEAM (under current conditions - whatever they are) in addition to the TEAM allowed by the operational freedom triggers.
After this lot of trials (four months), there is supposed to be a second trial in the summer (including the Olympics period) which may have similar or different triggers.
It may be that they'll give a few more dual take-offs using 09L/09R then as well.