"Imitation of the flight of the Humming-bird" is the accompanying comment that Sagreras makes to the piece on the score along with a dedication to his daughter Clelia for whom he wrote the piece. The work will be played at a rather fast pace...actually, most contemporary players pay it too fast... The faster a piece is meant to be played, the slower it has to be practiced and played until all the moves have been incorporated to a subconscious level.
Sagreras uses legato/slur (glissando) and a fast series of repeating sixteenth notes to achieve the effect of the fast rotating wings of this South American bird. 
Fast repeating notes can be achieved in several different ways:

• apply the planting "feel the string" with the tip of your finger prior to actually plucking the string. This will bring your fingers as close to the strings as possible
• use the a-i fingers combination if you want. These 2 fingers are generally identical in length.
• make you own exercise and use different fingers combinations on different strings. Speed must be increased gradually and I strongly recommend you use  a metronome during the study process of fast pieces to prevent you from playing fast ahead of time.  The fingers combinations are : m-i; a-i, m-a and p-i.
• Only when yo can play the piece throughout without mistakes, start working on speed.