What is pinging? https://www.investopedia.com/articles/active-trading/042414/youd-better-know-your-highfrequency-trading-terminology.asp
Pinging refers to the tactic of entering small marketable orders—usually for 100 shares—in order to learn about large hidden orders in dark pools or exchanges. While you can think of pinging as being analogous to a ship or submarine sending out sonar signals to detect upcoming obstructions or enemy vessels, in the HFT context, pinging is used to find hidden "prey."
Here's how: buy-side firms use algorithmic trading systems to break up large orders into much smaller ones and feed them steadily into the market so as to reduce the market impact of large orders. In order to detect the presence of such large orders, HFT firms place bids and offer in 100-share lots for every listed stock.
Once a firm gets a “ping” (i.e., the HFT’s small order is executed) or series of pings that alerts the HFT to the presence of a large buy-side order, it may engage in a predatory trading activity that ensures it a nearly risk-free profit at the expense of the buy-sider, who will end up receiving an unfavorable price for its large order. Pinging has been likened to “baiting” by some influential market players since its sole purpose is to lure institutions with large orders to reveal their hand.