How does solder flow upwards?

Quote from the video:
Quote from Youtube video: Inside the coupling you don't need a ton of flux. You just need enough such that the solder will flow upward due to capillary.

How do I know if my copper pipe is hot enough to solder?

Quote from the video:
Quote from Youtube video: You can test it out still not hot enough there it goes see it just melting in there.

Where do you put flux when soldering copper pipe?





Quote from the video:
Quote from Youtube video: Against the base of the cup a slight twisting motion ensures even coverage by the flux wipe excess flux from the exterior of the joint. Be sure to support the tube.