After 25 seconds (at 1124 feet height) bangs, vibrations and jerks were perceived in the aircraft. This was caused by a no. 2 engine surge. The engine was throttled down a little, but throttle control simultaneously changed to an automatic mode which increased throttle setting with altitude (Automatic Thrust Restoration - ATR). This in turn increased the intensity of the surging. The no. 1 engine surged 39 seconds later, but this was not noticed by the flight crew.
Using short words 
is there anybody here who would explain 'engine surge' to me please?
As usual, I have an idea what it might be, but it's rather vague. Especially on why it is such an undesirable occurence.