From the blog Barking up the wrong tree:
A threat is "I will do something that hurts you even if it’s bad for me." A warning is: "This serves my interests and is bad for you." It’s better to try to convert your threats into warnings. Threats are no good, warnings are.
In criminal cases, as with all things in life, obtaining a good deal comes only from a position of strength. This is why prosecutors give attorneys better deals than they give pro se defendants; I can credibly tell a prosecutor that a motion to suppress serves my interests and is bad for his case. You probably can’t.
And prosecutors hate to lose cases before trial; if I win the suppression motion, the prosecutor probably loses his case outright. Whenever you have a credible warning with at least a uncertain outcome as the consequence, your case is ripe for a good deal, a bargain.