▁with ▁r he ost atic ▁reg ulation ▁since ▁E . 6 2 6 , ▁tra ction ▁is ▁controlled ▁via ▁a ▁lever ▁( comm only ▁referred ▁to ▁as ▁man ig l ione ) ▁mounted ▁on ▁a ▁support ▁called ▁r on cola ; ▁this ▁support ▁has ▁several ▁not ches , ▁each ▁representing ▁a ▁portion ▁of ▁the ▁r he ost at , ▁plus ▁three ▁" end ▁combination " ▁not ches ▁and ▁two ▁" transition " ▁positions ▁( a ▁bigger ▁block ▁between ▁the ▁not ches ). ▁To ▁acceler ate , ▁the ▁driver ▁rot ates ▁gradually ▁counter clock wise ▁the ▁lever ▁not ch ▁by ▁not ch , ▁and ▁in ▁doing ▁so ▁the ▁various ▁r he ost at ▁contact ors ▁are ▁closed , ▁sh un ting ▁the ▁resist ors ▁and ▁lower ing ▁the ▁total ▁r he ost at ▁resistance , ▁also ▁allowing ▁more ▁current ▁to ▁the ▁mot ors ; ▁as ▁speed ▁builds ▁up , ▁the ▁counter - elect rom ot ive ▁force ▁low ers ▁this ▁current , ▁until ▁the ▁r he ost at ▁is ▁no ▁longer ▁necessary : ▁it ▁is , ▁then , ▁totally ▁ex cluded ▁( ob viously ▁if ▁in ▁tra ction ▁and ▁in ▁an ▁" end ▁combination " ▁not ch ). ▁This ▁safely ▁allows ▁tra ction ▁for ▁an ▁in defin ite ▁period ▁of ▁time , ▁within ▁certain ▁limits . ▁ ▁When ▁the ▁end ▁combination ▁not ches ▁are ▁reached , ▁the ▁driver ▁can ▁then ▁insert ▁the ▁single ▁sh unt ▁( one ▁for ▁each ▁combination ) ▁or ▁make ▁a ▁transition ▁to ▁the ▁next ▁combination , ▁re int rodu cing