That condition of Bid=Ask has now appeared with greaterfrequency which means shares available for sale from the retail float is drying up quickly and ITG Virtu Group is bumping up against diminishing returns.
Said another way, if they want to acquire more shares they will have to change their alogrithm and start accumulating more shares at higher prices, smacking in the face when Bid=Ask.
"collaring a stock price" or "swapping spit"