This post is on a very interesting condition. To put it in simple terms, Ischaemic preconditioning is a defensive technique adopted by those cardiac myocytes which have faced ischaemia in the past against future ischaemic episodes. There is another defensive procedure called Collaterals but that occurs in chronic ischaemia while Preconditioning occurs in acute conditions.
It was first observed while monitoring the ECG of a patient undegoing coronary balloon angioplasty. When the coronary artery was occluded by inflated balloon transiently, the ECG showed QRS distortions indicative of ischaemia but when it was occluded once again, ECG showed almost normal QRS waveforms.
Mechanism of Ischaemic preconditioning has something to do with Katp channels, the one blocked by Sulfonylureas like Glibenclamide, and also Adenosine receptors. But how exactly these components are involved, isn't clear yet.