What is the test for the defence of voluntary assumption of risk?

In the case of Hayes v. Nanaimo Shipyard Ltd. (1971), [1972] 1 W.W.R. 180 (B.C.S.C.), the headnote is stated as follows: Held that failure of defendant's servant to warn plaintiff, before taking his hand from the top of the machine, to prepare himself for the change of balance, amounted to negligence; the defendant's duty towards plaintiff was to take reasonable care to avoid injuring him, and for breach of that duty defendant was liable … The evidence did not support the defence of voluntary assumption of risk, nor could it be said that plaintiff was contributorily negligent in helping to move the machine, in the manner in which he did, or that he was negligent in failing to foresee that the defendant's servant might be negligent.