If it's an automatic then there is a button, "kickdown", beneath the gas pedal. When you floor it or press the switch, the transmission drops a gear. This will make the car accelerate faster, as it's in a lower gear and when you come off the gas, the automatic transmission re-engages as normal and shifts you up to a cruising gear.
Because the car's in a lower gear so it will do more rpm / mph. This is the reason for the surge. It's normal.
Gone but not forgotten: '87 UK 318... Carb madness!
Currently: '90 UK 525i completely stock
"Heaven for the climate, Hell for the company."