Edge Loop Slide
This allows you to make “cut” in a mesh, letting modify it further.
Ctrl-R
press “CTR+R” and hover your mouse (without click) over the edge of the mesh to be cut.
roll the middle mouse wheel to have multiple cuts
LMB
move around to choose a location for the cut(s)
LMB
to confirm
Note: RMB to exit cut
This is handy when you want to extrude only a certain part of a plane.