After writing the question Do Blue Origin's BE-3 engines need to run for 7 seconds to “warm up”?, I took a look at the YouTube video SSME ignition sequence (found in What are the small gas jets at the rims of the Space Shuttle Main Engines?) showing the Space Shuttle's main SSME engines, and they also take several seconds from the first appearance of burning exhaust to what looks like a stable, full thrust before the clamps are released and the shuttle begins to rise.
note: the connection is that both engines use LOX/LH2
What is happening here? How do the propellant flow rates and more importantly the resulting thrust vary versus time during this ignition sequence?