Mechanism of Competitive Two-Under Sometimes One-Under

In a competitive sequence, a Two-Under call may be made by way of what appears to be a one-under "call," in a sense. 

If you double an opposition two-under call, this works as a surrogate for a two-under call.  Hence, for example, a double of 4C could show one or both majors, or crunched cappelletti (any two suits), effectively.

A forcing pass of either a one-under call by the opponents, or a double of a one-under call from partner, also works, as it allows a double or redouble from partner to be the preference call for the higher suit.  One example is the runout of 1NT doubled, where a forcing pass enables that one-under 1NT to be escaped into a one or both minors holding.  So also, consider 2C-4D-?  A forcing pass here would allow opener's double to show spade preference and therefore would enable a cunched cappelletti forcing pass of the one-under 4D call.