There is not a lot of info available on "expert Mode".
I did find some.........

Expert mode in all cases. New 'mod' aircraft changes in the default mode also denoted by *.
Corrected weight of F4F-3
Bf109E1 to E3* prop pitch model changed. Reverted back to the standard 109 prop control. This was necessary because the ai does not perform properly with the manual prop. Players can still switch the prop to manual pitch (ctrl + 0 ) and should for the most re-alistic experience.
Slight Increase in Prop reduction for B17's, B24 & B29 to increase climb rate at altitude to bring them closer to documented climb performance times.
Modified maximum allowed revolutions for B-24J engine & Increased flaps down lift slightly. This solves the issue of AI take-off in overload conditions.
Modified MerlinIII* engines (for Spitfire MkI and Hurricane MkI) to reduce the gap between predicted il2 performances and reference historical performances (mainly RoC)
Modified Merlin XII* engines (for Spitfire MkII) to reduce the gap between predicted il2 performances and reference historical performances (mainly RoC)
Modified Merlin61, Merlin63, Merlin66 (18 and 25 lbs) for Spitfires Mk VIII and Mk IX to reduce the gap between predicted il2 performances and reference historical performances (mainly RoC)
Completely reworked GriffonII, GriffonIII and Griffon65 for Spitfires Mk XII* and XIV*
Corrected some weight issues on several planes such:
Spitfire MkII*
Spitfire MkXII*
Spitfire MkXIV*
Spitfire MkXVI*
(+revised il2 compare data included in the HSFX V7 Tools & Read me's folder. You will need to unzip & replace your current version to look at the revisions.)
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Fairey Fulmar takeoff & landing characteristics with Flaps down updated.
Ju52 3mg4e max and cruise speed revision.
EXPERT mode
Highly recommended that you try this.
Developed for SEOW but the same changes should be universally applicable.
This Switcher Option activates Aachens FULL FM's. (see full explanation below)
Custom ini's (see below, Ground and Ships) reduce load and we believe, make the ground and naval war more realistic. Tanks, Guns & Infantry perform more efficiently and historically in relation to each other. Outcomes of Naval battles are more as you might expect them to be, naval AAA uses effective rather than maximum ranges.
Ranges are usually less and reload times longer, so you can have more objects in missions or you can turn up the RoF settings.
The blast radius of larger Kilo bombs are equalized out so that IJN and Soviet bombs display similar blast characteristics to LW ordinance. (Not sure why they should have been so radically different, the power is similar, but the blast radius were radically different for the same amount of explosives.
Because this is its own unique net version we were able to enable a few extra opening Canopies for this option as the incompatibility that creates the smoke bug should not arise.
This is NOT compatible online with the Stock Game (you will not be able to join), it effectively turns HSFX into its own Net version. You will not be able to join Stock games with it, and stock games will not be able to join you. Host and Client both have to be running this Option with its own HSFXv5.0 net version to use this compatibly Online.
This Option allows us to explore a lot of new options without compromising the stock game, Oleg's FM's or the online community.
( note : If you drag the buttons or other files from this into your stock game version piecemeal because you like a specific FM etc, you will get a no boot.
This is deliberate. )