Now, what is a House Head or a Dynast?
Within a House there is always a leader, a House Head, that wields power over the rest of the members. A House Head has the power to legitimize bastards, call House members to war, and demand that they adhere to their Faith (refusal to convert will result in them creating a new Cadet Branch). The House Head also has inherent leverage on all House members born after they were made head, by virtue of getting a Hook on them (more on Hooks in another DD). They also gain passive prestige based upon the number of members in their House. House leadership follows the succession of the House Head, so that if you’re the leader of your House you will most likely keep that title on succession.
The Dynast, on the other hand, wields significantly more power than a House Head - with their power encompassing the members of all Houses of the entire Dynasty! The Dynast is always the most powerful House Head of a Dynasty, with leadership being updated on the death of the old Dynast. In addition to everything the House Head can do, the Dynast can also Disinherit/Restore Inheritance, Denounce/Forgive members of the Dynasty (which affects opinion in a major way), personally Claim titles held by Dynasty members, and make Dynasty members end wars they have against each other. All of these powers work against every member of the Dynasty, not just the House they’re a part of. The Dynast also gains prestige for every living member of the Dynasty. Being the Dynast is very powerful indeed, but you have to carefully weigh the powers against other benefits, as they cost Renown.