unfortunately, the movie Castaway perpetuated the myth that one needs a crack that goes through the thickness of the fire plow hearthboard in order to get more air into the contact trough. a crack that goes through the hearthboard, filled with tinder, would be doubly-impossible to use sucessfully. that makes about as much sense as adding tinder to your hand drill notch before you start drilling. tinder only gets in the way of fire-egg (ember) production by physically dis-allowing the char-powder to coalesce and increase in warmth until 800F or so is reached--spontaneous combustion forms the coal.
i've made fire-eggs (to later hatch into flame within a tinder nest) with Ca Buckeye on itself, Bamboo on itself, Giant Reed (Arundo donax) on itself--which works better than Bamboo probably because it's increased wall thickness, Seep Willow (Baccharis viminea) on CA Fan Palm, Seep Willow on Coast Redwood (Sequoia sempervirens), Seep Willow on Sotol, and Sotol on itself...