"slap the ask" means buy at the ask (or offer) price, instead of placing a limit order at or near the bid. Not that strengthening the bid is a bad thing!
But buying at the ask (or lifting the offer, etc) results in an uptick in price, rather than getting filled at the bid on a downtick.
It also puts professional shorts in a worse position since presumably when the price is rising, shorts are attempting to cover their shares as bids, rather than buying to cover at the ask and creating an uptick.
"Slapping the ask" pressures them into doing the same as price moves up and against them. It also removes shares for sale from the market, who might otherwise get tired of waiting as a limit offer, and hit the bid, filling the short who's attempting to cover.
There's probably a more concise way to explain it, but that's the idea.
