Sentence:  A guy can be an active and successful hot rodder for years without becoming even remotely involved with mathematical problems ; however , he will have a clearer understanding of what he is <WSD>doing</WSD> and the chances are he will be more successful if he understands the few formulas that apply to rodding . 
Meaning of the word marked with <WSD> tags:  carry out or practice; as of jobs and professions