I found this Q & A with Father Edward McNamara, professor of liturgy at the Regina Apostolorum Pontifical University, very interesting. I was unaware the liturgical norms governing Exposition and Adoration were this involved.
Q: Is it permitted to have adoration by simply opening the door of the tabernacle, and leaving it open for an hour? I was told that this was OK, and that it was in the "book." Could you please tell me what book, and where this came from? -- P.P., Miami Springs, Florida. Q: During solemn exposition of the Blessed Sacrament I have seen so many different signs of adoration that I am confused. Is it supposed to be the single genuflection, just as you would before the closed tabernacle? Or is it the long, solemn, single genuflection, more pronounced than before the tabernacle? Or is it the double genuflection (getting on both knees)? -- M.P., Columbia, Maryland
A: Before the Second Vatican Council, opening the tabernacle door was more common as a simpler form of adoration, especially in convents and oratories. In some cases the abbess or mother superior had special permission to open the tabernacle and expose the pyx.
Sometimes, especially in convents that practiced perpetual adoration, the Blessed Sacrament was permanently exposed in a small monstrance within the tabernacle or in a large monstrance above the tabernacle which was veiled from view during Mass and other ceremonies by an ingenious swivel door. This method, which is still used in some places, allowed for exposition to be interrupted and restored on a regular basis without recourse to incense or other ceremonies.
In a present parish context, or even in religious houses, exposition by opening the tabernacle is no longer necessary, since any minister who has the faculty to open the tabernacle, either in virtue of the sacrament of orders or by special permission of the bishop, may also place the pyx on the altar or place the host in a monstrance upon the altar.
While there is no express prohibition to exposition by opening the tabernacle, the directives of the liturgical books actually in force make no mention of this option and presume that both solemn and simple exposition is upon an altar.
Only an ordained minister may give Benediction. Another approved minister simply replaces the Blessed Sacrament in the tabernacle without ceremony when the period of adoration has concluded.
With respect to the genuflection: Since a genuflection is, per se, an act of adoration, the general liturgical norms no longer make any distinction between the mode of adoring Christ reserved in the tabernacle or exposed upon the altar. The simple single genuflection on one knee may be used in all cases.
However, some bishops' conferences have voted to retain the use of the double genuflection for the Blessed Sacrament exposed, and it would be required in these countries. In this case, those who make the double genuflection kneel briefly on both knees and reverently incline the head with hands joined.
Needless to say, the simple genuflection should never be reduced to a sudden spasm in the right knee. The right knee should touch the place where the right foot stood while head and back remain straight. The gesture of adoration should be performed with due pause.
When I was young a wise priest taught me to recite the invocation "My Jesus, I adore you in the sacrament of your love" so as to gauge a reasonable time to remain knee to floor. One could stay longer perhaps, but it is a fairly safe rule of thumb.