These two elements of signification and instrumentality are both essential to a Sacrament. Abbot Vonier writes on p. 20:
Whenever the sacramental doctrine is either falsified or deflected from sound tradition the cause has been this, that me, who ought to have known better, in one way or another ceased to visualize the double concept of signification and causation. The two concepts are strictly inseparable in this matter of the sacrament. The sacrament must be cause in such wise as actually to represent the past, the present, and the future; and it must be sign in such wise as actually to effect the thing which it proclaims.Keeping in mind that the Eucharist is an efficient cause, a tool, simplifies dealing with the charge that it is just some sort of magical rite, nor the priest acting the part of magician. (The very term "hocus pocus" is said to have been coined in mockery of the phrase, "Hoc est corpus meum.")
Given that the Eucharist is a tool or instrument of grace, it is clear in Catholic thought that the Person who uses the tool is none other than Christ Himself. And He uses it freely; the priest does not summon or conjure Christ with word or gesture. The tool is, remember, the very sacrifice of Christ, a sacrifice offered once in time that exists always in eternity. Since He has directed His priests (ordained priests in a particular way, but also every Christian baptized into His priesthood) to "do this," it is by His own will, not the priest's, that the bread and wine are transubstantiated.
The thought that the Sacraments are tools in the hands of Christ may also cause us to reverence them more than, perhaps, we already do.
As for the other essential element of a Sacrament, that it is a sign, meditating on this may help clean up some confusion about the fourfold presence of Christ in the Mass. Of the four -- priest, congregation, word, and Eucharist -- only one is sacramental, only one is a sign of Christ's passion that effects His passion. On p. 21, Abbot Vonier makes this "bold hypothesis":
If the priest at the altar brought down Christ from heaven in His natural state as a full-grown man, this would not be a sacrament at all, for the event would lack the very essence of the sacrament, representative signification.... It might almost be said that if at any moment in the sacramental process, Christ in His natural condition were to step in, the sacrament would at once be made meaningless.My point here is not to rank the various ways Christ is present in His Church according to importance, but merely to observe that there are various, substantially different ways He is present, from which it follows that the various ways cannot be treated indifferently or identically.