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Supergiant Fast X-ray Transients show rare outbursts comma while most of the time they are in a fainter state.The physical mechanism driving this behavior is controversial comma with two competing modelsdoublePoint quasi-spherical settling accretion and the propeller mechanism. Our analysis of archival XMM observations allowed us to select SFXT flares in an automatic way and to compare their timescales with the theory of magnetospheric instability in the NS magnetosphere. To get a statistically robust comparison with the models comma it is crucial to enlarge the number of flares.This will result in the determination of the dimensionless parameters involved in the Rayleigh Taylor instability comma impossible to obtain from the theory comma directly probing the physics of the NS magnetosphere.