What exactly is mainlining?

I've been growing for a long time now, but, I still learn something new everyday.
So I'm curious though, what exactly is mainlining? I've been seeing a lot of this word around lately since becoming a member on these forums and I don't understand it.

Also...

Merry Christmas & Happy Holidays!

I've been wondering that too. I've never seen the term until recently.

check out nugbuckets post in organics/subcool title is mainlining

https://www.rollitup.org/subcools-old-school-organics/542308-main-lining-thread.html
Should answer some questions lol, just a method of hard training to achieve symmetry in your plants.

Thanks, maybe I'll try that next time.