The basic practice is for the buyer to wait until everybody is just about to change contracts and then reduce the offer. The seller then has the option of agreeing to the new price or risking the sale falling through.
This dastardly practice last raised its ugly head in the murky days of the early 90s recession when, as is the case now, the market strong favoured the buyer.
For anyone that is interested the origin of the word Gazundering is that it is an obvious play on the word Gazumping. Gazumping is thought to have originated from the Yiddish word gazumph, meaning to swindle. The term was used as gangster slang in the 1920s and became synonymous with poker games.
Gazundering is legal and there is little you can do to protect yourself if a buyer is determined to play dirty.