This video demonstrate a few good techniques for smooth transitions between chords. My explanations below, taken from the video (far below), assume a general knowledge of scales and chords. The techniques used in the video are:
1. Using a pedal note. This is a note that remains the same through each chord in the chord progression, usually higher or lower in pitch than the chords in the chord progression. This could be a common note in the chords or the root note of the scale from which the chords originate. For example, in the key of C, if playing a C F G C chord progression, you could use C as the pedal note. This technique can be a good effect when used sparingly. Hearing the same note frequently or for long periods of time, gets boring quickly.
2. Chord inversions, or voice leading. For smooth chord transitions, it’s best to keep the distance between adjacent chords to a minimum. For example, if transitioning from a C (CEG) to an F (FAC) chord, recognize which notes both have in common and place those notes in the same position in each chord. Look at the difference in movement to get from one note to the other in these 2 examples of transition from a C chord to an F chord.
This is good voice leading with minimal movement between a C and F chord (C root position to F second inversion or C to F/C):
C -> C (no movement)
E -> F (semi-tone)
G -> A (2 semi-tones)
This example is more movement which will not sound as smooth between C and F chords (C root position to F root position):
C -> F (5 semi-tones)
E -> A (5 semi-tones)
G -> C (5 semi-tones)
3. Suspended chords. A suspended chord is achieved by moving the 3rd of the chord up or down. This is a useful way of providing a smooth transition between chords that have no common note. For example, to transition from a C (CEG) to a Dm (DFA) a suspended chord can be injected between the two as a short transition chord. In the following example, the 3rd in the C chord (the E) is raised to F to create a Csus4 chord. This means the 3rd has been suspended and replaced with another note.
C -> C -> D
E -> F -> F
G -> G -> A
In the above, there are now common notes between the C and the Csus4, and the Csus4 and the Dm chord.
4. Anticipation. This involves playing a note from the next chord, early in anticipation of the new chord. A suspended chord achieves this but this can also be achieved by anticipating any note in the new chord:
C -> D -> D
E -> E -> F
G -> G -> A
… or both can be used one after another for an even more smooth transition, first the sus chord, then the anticipation:
C -> C -> D -> D
E -> F -> F -> F
G -> G -> G -> A
As you can see, the C (CEG) chord, one note at a time, blends in to the Dm (DFA) chord.
5. Joining notes. This means instead of playing the same note twice, hold the note twice as long. In the first example where the chords were C played as (CEG), and F, played as (CFA) instead of playing the C twice, just hold the C.
E -> F
G -> A
The above is meant as a brief summary or reference. This is best understood by watching the video from “Hack Music Theory” which demonstrates these concepts very well.