Depending on the engine type, some starters are ‘direct-drive’, where the engine turns as soon as the starter turns. On these, you can have a shear pin in the connection that lets go when too much resistance is encountered. This should prevent the system from bending a rod or busting something due to hydraulic lock, and on these engines you can turn the engine over a set number of blades on the starter, before switching on the ignition.
On the B-25, the R-2600 engine has an inertia starter. In this setup the starter turns a flywheel, once this is up to speed you use the ‘mesh’ switch to engage the clutch, this transfers the energy from the flywheel to the engine by connecting the two. With this setup, a hydraulic lock would end up in a busted something as the energy in that flywheel will have to go somewhere. And you can’t include a shear pin, as it would probably go once every start whenever you hit ‘mesh’. The inertia system allows the starter to overcome a larger initial resistance in larger engines, with larger compression figures, but it introduces a risk as well as it is easier to bust something. Because of that, pulling the engine through a couple of times by hand is a sensible precaution. The number of blades you use for that is dependent on the prop gearing and the requirement to at least have every cylinder go through a complete intake – compression – expansion – outlet cycle. The first link in tbyguy’s post above explains this a lot better than I can.
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