The tremolo is a playing technique often used by 4 string banjoists to help give the effect of adding sustain to a note. This effect can add a lot of excitement to your playing and students request this probably more than anything else when learning to play tenor or plectrum banjo. This effect is used on many other instruments as well. You'll hear it very often employed by mandolinists as well.
The type of tremolo we are talking about here is a playing technique. This is not to be confused with the electric guitar effect called tremolo or the tremolo arm on some electric guitars (or even some early banjos). The electric guitar effect actually is a modulation effect that changes the volume in and out or side to side. The tremolo arm on some electric guitars is varies the pitch and is really vibrato.
tremolo picking or strumming that we are talking about here is the playing of a note fast and repeatedly so that it gives the impression of a single sustained note. Banjoists and mandolinists use it most often because the natural duration of a note played on these instruments is so short. When a long note or chord such as a whole note is in a tune, it is hard to keep the sustain of these long tones at the same volume without employing the tremolo.
First, you will need a metronome. You have to keep the rhythmic timing very precise. We're going to focus on a single string tremolo to start as oppose to a full chord tremolo. In the video below we will concentrate on playing an F chord tremolo. We will be using the down/up method of picking (this is where you strike the strings with a downward motion and then next strike the strings with an upward motion).
* towards the end of the video I did accidently say "triplet" a few times when I meant to say "tremolo".