What is a pro forma session? Here’s how the Senate is resisting Trump during their recess.

The Senate this week announced it will hold nine pro forma sessions during its August break, a move most are interpreting as a means to prevent President Donald Trump from making recess appointments in their absence.

The measure will also effectively keep Trump — who embarked on a 17-day “working vacation” in Bedminster, New Jersey, on Friday — from going after special counsel Robert Mueller, whose investigation into the Trump campaign’s Russia ties has been escalating in recent weeks.

What is a pro forma session?

The Senate typically has to approve high-level appointments made by the president. However, the president has the power to jettison that process when the Senate is in recess and appoint officials who have the full power of their position until the chamber reconvenes.

Sen. Lisa Murkowski (R-Alaska) — who recently defied Trump and her party’s leaders on health care — set up the pro forma sessions on Thursday, preventing Trump from doing that.

By holding the sessions — which can last less than a minute — the Senate is technically not taking a recess, thus removing the president’s power to make recess appointments. A senator will gavel in and gavel out every three days until the chamber officially reconvenes in September.

Trump has been rumored to be considering firing Attorney General Jeff Sessions, whom he’s publicly attacked several times recently for his decision in March to recuse himself from matters relating to the Russia investigation. Sen. Lindsay Graham (R-S.C.) has said that there would be “holy hell to pay” if Trump fired Sessions.

The move Thursday keeps Trump from being able to fire Sessions and replace him without Senate approval, which would be seen as the president going after Mueller; Trump has also left the door open to terminating the special counsel, which Graham said would mark the “beginning of the end” of his presidency.

Though most regard this week’s pro forma announcement as a protection against the president, a spokesperson for Senate Majority Leader Mitch McConnell said Friday the Senate “didn’t do it to block Trump.”

“To meet our constitutional requirement of meeting every few days, we’re doing pro formas,” he said.

It’s not the first time the Senate has invoked a pro forma session. In 2012, then-President Barack Obama made recess appointments to the National Labor Relations Board — despite the Senate not having officially recessed. The Supreme Court in 2014 ruled that Obama had overstepped his bounds in making the appointments.

“The Senate is in session when it says it is,” Justice Stephen G. Breyer wrote in the court’s unanimous decision.