I think you're referring to Presidential Signing Statements.
During the administration of President George W. Bush, there was a controversy over the President's use of signing statements, which critics charged was unusually extensive and modified the meaning of statutes. The practice predates the Bush administration, however, and has since been continued by the Obama administration. In July 2006, a task force of the American Bar Association stated that the use of signing statements to modify the meaning of duly enacted laws serves to "undermine the rule of law and our constitutional system of separation of powers".
Effectively, the President writes a signing statement to the executive branch saying "This is how we're going to interpret and implement this law" as he signs the bill. This clarification is a good thing if the bill is ambiguous or potentially conflicts with some other law or Constitutional provision. But when the directive reaches the level of "Don't bother doing this", it's effectively legislating. (Not that that is actually an example, as far as I know).