Here are some pictures that show you what the phrase “I forgot how to XXX” means.
As you can see, it means that somebody or something behaves in a very odd way. But you must be careful: This is not proper English, but Internetspeak. So don’t use it in your next test or I’ll have to mark it as wrong!
I also found out that the phrase works with literally anything you can imagine. Two more examples:
My favourite 🙂
See you tomorrow!
nalech 