Windshear doesn't always affect you. Never been flying on one of those days when the wind direction varies? Or flown close to hills or buildings, when the wind whips around them? That's windshear. It only affects you when you aren't expecting it. And it only affects your helicopter when it causes your airspeed to reduce below translational lift speed, and you don't realise it and add extra power. The same thing happens in a downwind turn if your airspeed reduces. The two are identical.