Basically it looks like this:
Tank is rendered immobile (can only turn) but it's fire rate is doubled (I think?) which was applied to units such as the Panzer Elite's Marder III, Panzer IV F1 "Stubby" and Hummel. In the last one's case it was to improve it's range as well, similiar to the KV2's "Siege mode".
Mods introduced this ability to the Firefly as well.
So idea is pretty straightforward, Firefly could go 2 exclusive paths, Tulip rockets or the Commander allowing it to "lock down" with the states described in the picture above.
That would allow for a more tactical decision for the Firefly and it's British user, instead of it being the useless heap that it is right now, in my opinion without directly "buffing" it. I also have no doubt that the M10 from the new commander will pretty much replace it in most games.
The same function can be applied to more AT guns, like the raketen as well for example, without again directly "buffing" said unit.
Cheers and have a nice day.