Incrementing Step. All we mean by this is that the gap between each letter gets larger or smaller with each step.

+3 +4 +5 +6 +7 etc

or

+5 +4 +3 +2 +1 etc 

AB DC HD NE (first letter gap increasing)

NE HD DC AB (first letter gap decreasing)

Switching sign. Sometimes the gap switches sign. By this we mean the gap could flip between going backwards and forwards.

+2 -2 +2 -2 etc

or combined with Incrementing step(above) you could get

+1 -2 +3 -4 etc.

This is explained in the video 

Two series in the one question. This is described at the end of the video. The example to the right can be thought of as two different sequences:

BC DE FG HI

XW UT RQ

so the next pair you have to find would actually belong to the second sequence. Watch the video - these things are easier to explain with animation. 

BC XW DE UT FG RQ HI

Answer: ON