The practice triangle is always something that I'm always trying to be aware of when I practise and when I teach pupils how to approach learning. I find that I make efficient practise when I use the practice triangle.
Most often, students tend to rush pieces that they are learning (too much speed), or they try and practice the whole piece in one lesson (too much duration). Slowing things down and/ or selection a shorter duration will improve the accuracy.
Duration: The length of the piece. The full duration would be the 'whole piece'. A part of the duration could be 4 bars, 1 bar, 2 beats, 1 beat... any duration you want to select.
Accuracy: How perfect you play the piece.
Speed: How fast you play or the correct tempo.
All of these things together would mean the performer can play a piece 'perfect'.
In practice, you should select two parts of the triangle before attempting all three.
Practising with duration & accuracy: This would mean that the speed is taken out of the equation. Slow a piece down and you'll find that you can play it accuractely and you'll be able to through more of the piece.
Practising with accuracy & speed: This would be taking a very short part of the piece (duration), lets say a bar of music that is very difficult. Practise the notes in the first beat only. You should be able to play it with good speed and accuracy before increasing the duration.
Practising with speed & duration: This would drastically reduce the chance of having accuracy. It is fun sometimes to attempt a piece by playing all of the way through at the required speed. It may not sound good but it's an indication of how fast you can learn a piece within a short period.