What received constitutional understandings did Pitt break in 1784?

Before Pitt:

1) A ministry (“government”) must command the support of the House of Commons,

2) because Commons must yearly pass the budget and mutiny act (see footnote below).

3) Without the budget the Ministry cannot afford an Army to repress Britons.

4) Without the mutiny act the Ministry cannot lawfully control an Army to repress Britons.

Pitt was in a struggle with Commons over the course of Britain. Pitt was in a struggle with Commons over Pitt being pre-eminent Minister of a Ministry appointed by the King. Commons had numbers over Pitt.

What Pitt did:

5) Pitt went to the Crown and got an election. This vacated the Commons.

6) Immediately Pitt could rule as a caretaker ministry.

7) He hoped the newly elected commons would support him.

8) The electors therefore had the power to determine who the commons would be, and thus who the ministry would be.

9) This power to elect the commons was an even greater force than the threat of not passing budget (also called "supply") and mutiny.

In Summary:

Pitt refused to change his ministry to meet the will of Commons. This was his action. He supported his action by calling an election.

To paraphrase Brecht: If government is upset with the House of Commons, it must elect another one.

Footnote: the mutiny act and budget give the commons assembled an ability to control the army (ie: shoot down a popular revolt). The sovereign is the party who commands that capacity. Pitt transferred sovereignty from the Commons to the Electorate. Note: while not constitutional precedent the unwillingness of the Revolutionaries to allow elections under the interregnum was evidence that commons, not the electorate, was functionally sovereign in the interregnum. The ministry’s manner of dealing with a hostile commons in the interregnum was unique and did not establish a precedent. But the idea of an evil ministry and commons control over the army was cemented by these events.

To paraphrase: Prime Minister Pitt's government received a vote of no confidence in Parliament. but Pitt enjoyed support from both the King and the House of Lords, so he ignored the house's vote, refused to resign, and instead dissolved Parliament in 1784. Pitt made them stand for re-election. The voters believed Pitt was honest, and Pitt's corrupt opposition favored Pitt's supporters in the election. So in 1784 Pitt remained in office and proved "the sovereignty of Parliament was subordinate to the political sovereignty of the nation"

Pitt The Younger
So as to reduce the power of the Opposition, Pitt offered Charles James Fox and his allies posts in the Cabinet; Pitt's refusal to include Lord North, however, thwarted his efforts. The new government was immediately on the defensive and in January 1784 was defeated on a motion of no confidence. Pitt, however, took the unprecedented step of refusing to resign, despite this defeat. He retained the support of the King, who would not entrust the reins of power to the Fox–North Coalition. He also received the support of the House of Lords, which passed supportive motions, and many messages of support from the country at large, in the form of petitions approving of his appointment which influenced some Members to switch their support to Pitt. At the same time, he was granted the Freedom of the City of London. When he returned from the ceremony to mark this, men of the City pulled Pitt's coach home themselves, as a sign of respect. When passing a Whig club, the coach came under attack from a group of men who tried to assault Pitt. When news of this spread, it was assumed Fox and his associates had tried to bring down Pitt by any means.[28]

William Pitt in 1783 Pitt gained great popularity with the public at large as "Honest Billy" who was seen as a refreshing change from the dishonesty, corruption and lack of principles widely associated with both Fox and North. Despite a series of defeats in the House of Commons, Pitt defiantly remained in office, watching the Coalition's majority shrink as some Members of Parliament left the Opposition to abstain.[28]

In March 1784, Parliament was dissolved, and a general election ensued. An electoral defeat for the government was out of the question because Pitt enjoyed the support of King George III. Patronage and bribes paid by the Treasury were normally expected to be enough to secure the government a comfortable majority in the House of Commons, but on this occasion, the government reaped much popular support as well.[29] In most popular constituencies, the election was fought between candidates clearly representing either Pitt or Fox and North. Early returns showed a massive swing to Pitt with the result that many Opposition Members who still had not faced election either defected, stood down, or made deals with their opponents to avoid expensive defeats.