http://www.kwanumzen.org/primarypoint/v02n4-1985-fall-dssn-bmzm-theswordthatkills.html

There are two formal aspects to Zen practice: the killing sword and the sword that gives life. The killing sword means, how do we give ourselves to the situation? How, without repressing, do we let go of our condition, opinion, and situation and really offer ourselves to what's going on in the moment? For most of us that requires a fair amount of hard work. Sometimes it's painful and difficult. If we try to live that way all the time, only being "good" or only taking certain roles, most of us find that it doesn't work. We become brittle, irritable, out of balance. We do many things with such a charge behind them that we put ourselves in precarious situations.

The other aspect is the sword that gives life: the experience of empowering ourselves, doing things which come naturally, that we love to do and find fulfillment in. When we do too many of these things, most of us get a certain softness or flatness. There's no keen working edge to our practice. It's hard to believe in ourselves if we go too far to that side.