Forward Intention has been defined by inference only, and demonstrated visually on this site in 3 different places: Intention Based Dancing, The Second Step Problem, Leading Technique, not to mention being discussed ad-nauseam in nearly every single Tango Topic in some way, shape, or form.
However, again, for the sake of clarity it can best be defined as a “little lean forward“.
To be more precise:
Where the Dancer (Lead and Follow engage in Forward Intention), engages their toes. Spreading them out inside their shoe/sock, and then pressing them into the floor as the phalanges (the toe bones) begin to take weight. There is a softening the knees with a little bit of downward compression, as well as a tightening of the quads to hold a stabilizing position. At this point, the Tibia & Fibula bones are allowed to float forward over the Talus bone (Ankle joint) resulting in about 2 to 4 degrees of forward tilt against a vertical line.
This is a “a little lean forward”.