You know what I've been playing spades since I was 9 and I'm as confused as you. Reneging means playing a card from the wrong suit, i.e. playing a spade even though hearts was led and you have hearts. This happens very rarely and it's almost always a player error, and it's almost never done in relation to the bid. There's no strategy that involves reneging, it's just a penalty when someone sees it happen. (Someone always sees it happen. It's hard to renege without someone at the table knowing what cards should be left.)

It's possible OP was talking about "setting" the other team, which means tricking the other team into bidding overconfidently so they fail to take enough tricks. (This results in a big score penalty for the team going set.) Sandbagging (deliberately underbidding) can definitely be used, and legally, to make the other team go set.