No-poaching agreement is, presently, a contract between the recruiting agency and the firm. Soon, hopefully (but improbable), there will be a 'gentlemen' agreement between two companies who will not poach into the other company. This is when the companies are doing the similar business. If one goes through the IT industry HR retention, one sees maximum attrition!
Many years back, in the airline industry in India, there was lots of poaching between airlines - this was when many airlines started up (2005-06 - SpiceJet, Kingfisher, Go, Indigo, Paramount) and the market was deficient with skilled manpower (including Pilots). There was lots of poaching till the regulator (DGCA) stepped in and issued a rule that Pilots should give a six month notice before leaving an airline for joining another.
Rajusiachen