To begin with, a Cadenza is not something a string arranger would usually write into an average 3 minute pop song or rock track but they can be added into other styles of music and apply to any instrument (not just strings!). A cadenza comes from the word ‘Cadence’ (a cadence is an ending either to a phrase or even section of a piece) and is essentially an ad libbed passage in a piece of music (usually at a Cadence) where a solo instrument breaks out from the rest into a flourish. In the baroque era, a singer would often embellish a cadence with an improvisation (normally near the end of an aria). Later, in the classical period the cadenza developed into an improvised solo at the end of the recapitulation section and before the coda of a first movement (normally a concerto). In this form, the soloist would take themes from the piece and develop them, often changing the form into something quite different yet still related to the original melody. Cadenzas became increasingly virtuosic, allowing the performer the chance to really demonstrate their skills on the instrument - and in many cases became quite a few minutes long. Nowadays, a cadenza is less commonly improvised and more often carefully composed before the performance to cleverly capture themes from the piece and gain the maximum impact whilst still feeling ‘improvised’. The performer has the complete attention of the audience during a cadenza so it’s important to ‘get it right’ and most cadenzas are carefully prepared beforehand.
In a string arrangement for a song, the length of the track will determine how long a cadenza might last but in modern pop or rock music it now takes the form of an instrumental break - where the vocalist and other instruments takes a pause and one of the instruments comes to the fore with a solo phrase or quick flourish, possibly only lasting a few seconds. An appropriate section could be at the end of the introduction, before the first verse begins.
So how does a cadenza differ from say, a guitar solo? In a guitar solo the beat or pulse of the track continues underneath - sometimes the same chords repeat, allowing the guitar to float over the top with improvised runs. In a cadenza, the beat of the music stops completely (like a pause), allowing the instrument to provide a fill in until the music starts where it left off. A cadenza is usually a solo instrument, but there’s no reason why a whole section of instruments couldn’t play one!
