Starting problems?

I bought this piece of kit RAVPower Portable Car Jump Starter External Battery Pack a while ago and have found it very useful on several occasions (my 911 suffers badly from battery drain), but today it really excelled itself.

The battery on my 550 was dead, as in stone dead, not a sign of an amp, volt, or even a millivolt - all electrons had departed for pastures new! Despite that on the manufacturer's website it says that it will start most engines less than 5.0 litres, it made short work of my Maranello - I connected it up, more in hope than expectation, and after a couple of tries and a little bit of clicking, the starter turned over and the engine fired into life!