Samkhya's account of the physical world proceeds by successive addition rather than by five independent tracks. Vayu, the air element, is not described as carrying only sparsha, touch; it is described in later systematic summaries as carrying sparsha together with shabda, sound, the quality of the tanmatra before it, because grossening accumulates qualities as it proceeds rather than discarding the ones already established. Tvak's tanmatra, sparsha, is therefore the second link in a chain that begins with shabda and ends five steps later in prithvi, earth, which by then carries all five qualities at once.
This cumulative structure is what makes tvak's specific position, second among the Jnanendriyas, meaningful rather than arbitrary. An organ tuned to the second tanmatra corresponds to an element, vayu, that has one more layer of density and one more inherited quality than akasha, the element linked to shrotra's tanmatra shabda. The correspondence between organs, tanmatras and elements is exact at every step, which is part of why Samkhya's twenty-five tattvas are usually presented as a single continuous account rather than as several loosely related lists.
Tvak also illustrates a point that applies across all five Jnanendriyas: none of them is described as producing sensation independently. The organ registers its tanmatra, manas synthesises the result with the output of the other four organs, and only at that later stage does anything resembling a unified tactile experience, warm, rough, pressing, arise, well downstream of tvak's own narrow and specific operation.