Ta Toybox

I knew 'Toybox' would have the answer! A sap is:

Now for some historical garb, a Sap  is a trench in front of   another  to
allow the front rank to kneel and fire whilst the back row in the trench
behind reload(remember they used back powder muskets in those days and it
took about about 20-30 seconds to reload. While the front rank were
reloading the back rank would fire. This harked back to the battle of
waterloo when the Poms(as you call them) did so well against Napoleon. 

Thanks, Steve.
:-)