A year ago, LHR introduced a new technology in its Terminal 1 and 3:

‘Positive boarding’ will help reduce airlines’ last minute searches for passengers or their bags, as well as give travellers more accurate information to help them smoothly through their journey.

Think of it as a way to tell airlines if you're going to make the flight or not. In other words, 'positive boarding' is a marketing term for passenger tracking.

When you scan your boarding pass at the automated gate, the PASS system sends the data to the airline, which can verify if you're in the correct terminal and how much time you have to reach the gate. The airline can then determine if you're too late and if they should offload your bag, avoiding further delays. 

Positive boarding (image courtesy of Heathrow Airports)

Messages can be displayed to warn you to rush toward the departure gate or to return to a check-in gate if you're too late (the ideal scenario would also include warning integration with the airline mobile app or a SMS gateway, but fragmentation is high there; that shall be the topic of another post).  I've personally never seen it in action (I'm a regular at LHR Terminal 1 and 3 but never late I guess),

Each minute spent on the tarmac for a plane in LHR costs around $115. Offloading a luggage often means losing a take-off slot, making the aircraft go at back of the queue. That can add up to 20-30 minutes. You do the math.

Data mentioned by Heathrow in 2013 seemed to show that more than 40% of Virgin Atlantic passengers were at risk delaying a departure. Adding Terminal 1, 3 and 4 together, late-running travelers were apparently responsible for 50,000 minutes of delay (with security sometimes not properly staffed or information displays being occasionally late to publish information, blaming passengers alone is evidently too easy and not my intent at all here).

Besides Terminal 5 which already had a similar technology thanks to British Airways and after Virgin and United, the 'Positive Boarding' technology has now been integrated with Malaysia Airlines systems. No word on the actual efficiency of the system since last July though. 

The current (soft) opening of Terminal 2 should see this adopted more widely.