How to force run a method triggered with a series of keys in a game?

Considering that this is the RE forum, this is how I would find the function.

1. Place a conditional break point on the input handling code. Depending on the game it might come from something like getchar() or be handled in a Windows message loop (assuming x86 Windows here since you mentioned OllyDbg).
2. The condition for the breakpoint would be when the input data equals 'j'. You might get lucky and find a cmp/jmp (or similar) sequence for 'j' or 'J' in the disassembly, then you could just set a normal break point there.
3. Run the game, and execute the combination on the keyboard.
4. When the break point hits, you can then single step and keep track of the call statements that are hit, one of them is likely to be the function you care about.

Assuming this game is a bit more complex than a 8bit console game I'm afraid hooking getchar() and the likes won't do you much good.

With Windows windowed/full screen games, input is received by handling window messages, usually in a message loop.

In such cases, each window has its own message processing loop, in witch it constantly calls GetMessage() to receive events it should react to, such as key presses. GetMessage()s first parameter is a pointer to MSG structure which it populates with the next message after removing it from the queue. Go here for a message loop example, here for GetMessage() documentation, and finally here for the MSG structure documentation.

In that case you should place breakpoints on:

1. GetMessage
2. PeekMessage

After you've got to the message loop, you'll probably encounter a medium sized switch-case handling the different message types. That switch-case should read the MSG structure's message member for message types that belong to the keyboard. most commonly those are:

1. WM_KEYDOWN
2. WM_KEYUP
3. WM_CHAR.

Once you've found the case handling those types, you should be pretty close to the function you're looking for. That case will most-likely do a few lines for code worth of parameter preparations and then call the function you're looking for.