Tough one, being the person who wants to acquire the knowledge, you can use these methods:
Take the next maintenance task for any system you do not know:
A small ticket is your way into understanding at least the entry point of a system.
Pair program with someone who does know the system:
You drive and let them steer you into solving the problem or implementing the system.
Ask your teammates / manager about the system:
They will probably give you an overview and show you the essential parts of the program / system / interaction points.
Play with the code and see what happens:
Make sure the author has taken into consideration that production resources should be disabled by default; no one wants DEV emails going to customers..
Now, as the person who has the knowledge:
Write tests for your team:
Free documentation along with making sure the system works!
Create a wiki and put important processes there.
I usually use a markdown editor like Ghosterwriter and using Pandoc to convert the markdown to whatever format is needed.
Be willing to share the knowledge and share multiple times if you must:
As a team member, it's your job to empower your teammates. Plus, you may need them to know if you ever want to go on vacation!!
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Tough one, being the person who wants to acquire the knowledge, you can use these methods:
Take the next maintenance task for any system you do not know:
Pair program with someone who does know the system:
Ask your teammates / manager about the system:
Play with the code and see what happens:
Now, as the person who has the knowledge:
Write tests for your team:
Create a wiki and put important processes there.
Be willing to share the knowledge and share multiple times if you must: