Release

This is a technique that often accompanies bending, because it consists of playing a string previously pulled by a finger on the left hand. Immediately after being played, the string is released until it returns to its original position. The sonorous result will therefore be that of a falling note of many semitones that were formed by the preceding bending. In a score this type of situation is shown with one note in parenthesis that drops down on the following note, not picked but created by the sound of the departing note, that was on a taut string thanks to the bending. At times, under the curved line that connects the two notes, the letter "R" can be present, which indicates the release.
The biggest difficulty in the use of the release is surely the fact that the string must already be pulled when we play the note. This requires a good mastery of bending, because it is necessary to bring the string to the correct bending before playing it.
In the example below, the release is developed between the first note previously played (9 in parenthesis, which is the E note) and the second note of the 7th fret (D note). To perform this release, push the note at the 7th fret up a tone without playing, in order to prepare the starting intonation. Play the note and release the finger to perform a harmonic descent in the sound of the note: basically, you pass from E to D in a fluid manner. The velocity of the release depends on the speed of execution.

But the release can also be used by connecting it to the bending, as already mentioned in the special section devoted to bending. Once played, the note is "pulled" to the intonation given by the bending, then release the tension of the string until it returns to the point of origin. Musically this is resolved in a single ligature between a minimum of three notes: the first (starting) is connected to the second note with bending, which is connected to the third by the release. In this specific case the term release bend can be used.

Vibrato

Manipulating and making a note expressive is very important when trying to achieve perfect musical execution. Frequently there is one note that you need to work on that must be kept long. This can be taken care of by using some manual effects. One of the most commonly used is surely the vibrato, a technique that allows the use of some rotatory wrist or finger movements to obtain a small tonal variation in the note, usually called vibrato.
This technique is probably the most difficult to execute, because the vibrato is a sign of expression. Many years of practice and a lot of experience are needed to obtain a good vibrato, mostly because of the fact that musical sensibility is in constant growth as well as technical growth. It is not necessary to be a master of technique to have a good vibrato, and this can be understood by observing (and listening) to guitarists like B.B.King, Albert Collins, etc. Well these guitarists, using common techniques, are able to express notes with their fingers that many technical guitarists don't have. In fact many guitar heros have spent many hours studying and understanding how to make a single note "sing".
This introduction was necessary to explain the use of the vibrato and how it is of fundamental importance in playing the guitar. Now we will learn the basics of this technique starting with the placement of the left hand.
First of all we can confirm that there are two principal ways to perform the vibrato, one more classical and the other more modern. These can be personalized to your own style.
In the classical vibrato, the finger of the left hand that performs the vibrato makes a movement parallel to the strings. In fact the finger moves slightly to the inside of the fret, continually alternating the direction (regularly back and forth) and maintaining the line of the string that you are playing. In most cases, the speed of the movement is subject to the timing of the song. The result of this movement involves a variation in the intonation of the note, which is usually very slight (in microtones), and it assumes the aspect of a "flickering" of that note. The classical vibrato is mostly used in classical, folk, and ethnic music, and is the least common of the two techniques.
The modern vibrato is much more common and is sometimes called the rock or blues vibrato. Generally, when people speak of vibrato they intend this type of technique. In the modern vibrato the finger moves almost perpendicular to the string, and therefore parallell to the fret, causing a microtonal variation of the note, but with more emphasis with respect to the classical vibrato. In some cases you can get to a fourth of a tone or more, especially if you are feeling particularly energetic: listen to guitarists like Steve Lukater or Gary Moore and you will understand what a very powerful vibrato is, particularly if performed with a good distorted sound...
Although the two preceding cases show the most common ways to use the vibrato, you should know that there are other more particular systems to obtain the vibrato. As we said above, these systems are mostly characteristic of the style of a given guitarist and in many cases you can only understand how its done by analyzing and studying the style and technique of that guitarist. For example, a vibrato normally used by Steve Vai involves the movement of the finger on the string in a circular pattern. The finger, in fact, seems to make a circle on the string, resulting in a particularly sweet and light vibrato, which is often used on long and expressive notes.
It should be pointed out that the main performer of the vibrato is not the finger that presses the string, but the left wrist. In fact the movement of the finger (parallel or perpendicular) actually comes from the fact that the wrist rotates on its own axis to obtain the movement which is then transmited to the finger. Unfortunately, this is easier said than done. The only suggestion I can give is to practice, practice, and practice some more. Initially, the effect will be very scarce (even scholastic), but this is normal. It is important to use the vibrato in various situations, in order to "train" the wrist and fingers.
Here is a trick to understand the intonation and the precision of the vibrato. Get a tuner (preferably chromatic) and play one note on any fret so you can see it on the display. Now play again that same note and try to vibrate it using one of the techniques analyzed. If the vibrato is correct, the needle (or the lights, depending on the type of tuner) should move slightly to the right by a few hundreths of an intonation. This movement should rhythmically fluctuate depending on the speed set during the execution of the vibrato. Try to get a succession of constant and regular movements.