What Z said.
I have little practical experience with them as they were not in the game when I was playing it on the table top.
The Germans start the game with one (should not use it for Poland) and get another at the start of 1940. They are expensive to build and consume a fair amount of oil, but can be devastating if used in the right spot.
Essentially, you have to name a HQ as the beneficiary and there are some different options in terms of what you can use the offensive chit for. For the Germans, they would probably use them to perform a "super combined" (essentially, you can move unlimited units at once, including naval, air, and ground), or to turn all face down HQ's face up or in your case, probably a land chit. The land version allows 1 HQ to double the combat value of ground units equal to double its org value (so Rundstedt can do 8 ground units) within its command range. That means that stack of panzers that were worth 16 points are now worth 32 points while using 2 of your doubles. You can also use it multiple times on the same unit, so lets say the stack of panzers caught some hapless Russian 3 strength infantry corps in the open. Normally the panzers could not overrun, but doubling the stack to 32 allows them to overrun the 3 unit. The doubling effect then goes away, so they would have to be reselected to be doubled again. At the end of the current impulse, the HQ that was used will get flipped over, so you want to burn it on reorgs before that happens.
At any rate, think of the implications of doubling 8-10 German units with a chit when using one of the better HQs like Rundstedt or Manny, etc.