The soldier in charge would command, after each firing, that the rank on the scaffold step down and be replaced by the rank that had just reloaded, thus alternating ranks and sustaining the rifle fire.
( ( ( The soldier ) ( in charge ) ) ( ( would ( ( ( ( ( ( command , ) ( after ( each firing ) ) ) , ) ( that ( ( ( the rank ) ( on ( the ( scaffold step ) ) ) ) down ) ) ) and ) ( be ( replaced ( by ( ( the rank ) ( that ( had ( just ( ( reloaded , ) ( ( ( thus ( alternating ranks ) ) and ) ( sustaining ( the ( rifle fire ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN soldier)) (PP (IN in) (NP (NN charge)))) (VP (MD would) (VP (VP (VB command) (, ,) (PP (IN after) (NP (DT each) (NN firing))) (, ,) (SBAR (IN that) (S (NP (NP (DT the) (NN rank)) (PP (IN on) (NP (DT the) (JJ scaffold) (NN step)))) (ADJP (RB down))))) (CC and) (VP (VB be) (VP (VBN replaced) (PP (IN by) (NP (NP (DT the) (NN rank)) (SBAR (WHNP (WDT that)) (S (VP (VBD had) (VP (ADVP (RB just)) (VBN reloaded) (, ,) (S (VP (VP (ADVP (RB thus)) (VBG alternating) (NP (NNS ranks))) (CC and) (VP (VBG sustaining) (NP (DT the) (NN rifle) (NN fire))))))))))))))) (. .)))
The commanding soldiers just stayed and fought themselves with no rotation. 
( ( The ( commanding soldiers ) ) ( just ( ( ( stayed and ) ( ( fought themselves ) ( with ( no rotation ) ) ) ) . ) ) )
(ROOT (S (NP (DT The) (JJ commanding) (NNS soldiers)) (ADVP (RB just)) (VP (VP (VBD stayed)) (CC and) (VP (VBD fought) (NP (PRP themselves)) (PP (IN with) (NP (DT no) (NN rotation))))) (. .)))