Tehnically, in recording, a Tilt EQ contains two shelving EQ bands that rotate around a common turn point. Turning the control counter-clockwise from the center detent raises bass and lowers treble. Turning the control clockwise from the center raises treble and lowers bass. In that application the turn point is typically sweepable. It is a handy tool to handle tracks that exhibit a profound bias towards one end of the spectrum or the other.
In this Marshall case we need to think back to the "four-hole" original JTM and JMP amplifiers that had... wait for it... four inputs, serving two channels.
There was a hack that guitarists found to get more flexibility out of the old heads: You would plug your guitar into the hotter input of the Normal channel and plug a jumper cable from the lower input of that channel to the hotter input of the Bright channel. By doing so you could blend the channels to taste, blending the deeper sound of the Normal channel and the brighter sound of the Bright channel. When they were equal there also was a slight boost.
The Tilt control on the Origin Series does just that, allowing you to blend between a fuller sound and a brighter sound. The pull boost (also foot switchable) adds the little bit of boost on demand.
Bob