My partner and I use this method and have rarely had an "accident", and never a conception.
There is semen that is "leaked" before ejaculation, so that can be a trap. Some men leak semen more than others I think.
Most men don't have enough control to make this reliable, so I think it does depend on your partner and how able he is in recognising the signs and feelings during sex. Would he be able to recognise if it hadn't worked?
I think it is very common for people to conceive with this method.