Often used to describe something that works well above the average or the norm.
From the days when the cost of an average rocket (firework) was 2d ( less than a penny).
Therefore one costing two shillings or "two bob," (12 times the cost of the average)would be considerably more impressive.
" That weans cart/bogie (home made vehicle made from a plank and two pairs of wheels, commonly from a pram.) goes like a twobobrocket!"