In jazz music theory, the cadential chord progression from iv7 to I, or flat-VII7 to I has been nicknamed the backdoor progression. This name derives from an assumption that the normal progression to the tonic (V7 to I, or the authentic cadence) is, by inference, the front door. It can be found in popular jazz standards in such places as measures 9 and 11 of My Romance or measures 10 and 28 of There Will Never Be Another You, as well as Beatles songs like In My Life and If I Fell. It can be considered a minor plagal cadence in traditional theory.
The flat-VII7 chord, a pivot chord borrowed from the parallel minor of the current key, is a dominant seventh. Therefore it can resolve to I; it is commonly preceded by IV going to iv, then flat-VII7, then I.
(Courtesy of Wikipedia)
In My Life: (verse / key of A)
|A / E /|F#m / A7/G /|D / Dm /|A / / /|
Dominic Pedler in his Songwriting Secrets Of The Beatles explains it this way: "A favourite Beatles manoeuvre gleaned from a strong songwriting tradition was to play a IV chord - but switch it from a major to a minor triad before returning to I. The same root movement applies but now the progression is F-Fm-C. This idea dates back most famously to Cole Porter's claassic Evertime We Say Goodbye, where this hybrid cadence is even cued by the immortal line: How strange the change from major to minor..."
Everytime We Say Goodbye: (A secton / key of Eb)
|Eb / Ebo /|Abm6 Bb7 / /|Eb7 / Ab7 /|Abm / / /|
|Eb / C7 / |F7 / Ab Bb7 |Eb7 / Ebo /|/ / Bb7 /|