I haven't taken one apart but I can tell you a little about the check valve design.
The ball is seated on seat due to the spring and forms seat area such that when pressure forms on the area produces a force. Pressure (P)= Force (lb)/area (in^2). That why pressure is PSI or lbs per square inch. So F=PA and the pressure to consider is the net pressure (delta_p) upstream minus the downstream.
The spring also produce a force on the seat through the ball equal to F=kX. k is the spring constant of the spring (lb/in) or how much force with each unit of compression. x in the poppet closed state would be the initial spring deflection. x in the poppet open condition would be the additional deflection (just the total spring load at that amount of compression)
In theory when the poppet is closed, if the pressure produced by the spring P=(kx)*(seat area) is equal to P (the fuel pressure minus the cylinder pressure), the valve will stay closed. That would be the non flow line pressure trapped between the injector and the poppet as the line pressure bleeds off due to the closed injector. When the injector opens and the line pressure goes up, that total pressure would need to fully overcome the spring in the full open position as the poppet finds a sweet spot where the pressure against the seat forces the ball open and where the flow is moving around the ball to hold it off the seat.
Now you can go take some measurements, and measure the load as you compress the spring at workbench and probably get a pretty good idea of how that poppet works. Fun stuff : )
