In transitive terms the difference between underdrag and underdraw

is that underdrag is to exert pull on the underside of (something) while underdraw is to cover or line the underside of a structure (such as a floor) with plasterwork, boarding or other such treatment.

As a noun underdrag

is the tensile stress exerted by an overthrust rock mass upon overlying rocks which themselves are not under compressive thrusting stress and therefore tend to be stretched and broken by normal faulting.