T-Squared comes from the concept of multiplying 'T-Shaped' people by 'T-Shaped' people the way iron sharpens iron. T-Shaped people have a wide variety of skills and deep expertise in several disciplines. These are the 'A'-players, the Thought Leaders, the Executers who have the rare ability to get things done.
The concept of T-shaped skills, or T-shaped persons is a metaphor used in job recruitment to describe the abilities of persons in the workforce. The vertical bar on the T represents the depth of related skills and expertise in a single field, whereas the horizontal bar is the ability to collaborate across disciplines with experts in other areas and to apply knowledge in areas of expertise other than one's own.
The earliest reference is by David Guest in 1991. Tim Brown, CEO of the IDEO design consultancy defended this approach to résumé assessment as a method to build interdisciplinary work teams for creative processes.
Simon Sinek discusses the power of Why people do things in the powerful TEDxTalk below: