The basic sedevacantist view is that non-Catholics became "Catholic clergy"; then in order to get rid of clergy, they changed the prayers (form) of sacraments that create clergy, so that no clergy are created when these "new sacraments" are administered.
For example, Catholics believe an eligible man may become a priest when a valid bishop says specific words to the effect of "I ordain you as a priest" along with the necessary matter and intent of the sacrament of holy orders.
The idea is tha that basically the equivalent phrases like "I ordain you as priest" have been rendered ambiguous, leading some to believe these people do not become priests at all when these "new sacraments" take place.
For example, they might just not say "I ordain you as priest". Or, what I think they did, was say something ambiguous, like "I advance you, as if you had priestly powers, to act as a priest before God" which muddies the waters. Did the person become a priest or not (because they're not saying they ordain them clearly, but using unclear language)?
Now many of the sedevacantists have outright argued the rite of consecration that creates bishops is invalid. Sacraments are either judged to be valid, invalid, or doubtfully valid. A doubtfully valid sacrament is remedied by the sacrament being conferred conditionally: that is, the form of the words says something like "if you didn't receive this sacrament validly, I now validly confer the sacrament". So my argument is that these sacraments are more "doubtfully valid" rather than outright invalid.
But anyway to recap, the idea is that non-Catholics tried to take over with Vatican 2, make the sacraments invalid so there are no clergy, and then Catholics are effectively cut off from clergy.
Do you think this process took place or what do you make of the alleged "tampering" of sacramental formula and its implications?
(This is also perhaps an argument for sedevacantism coming from the other direction: we see the sacraments have been changed and "tampered with", ergo this suggests Catholic authority wouldn't have done this and that Vatican 2 must be rejected as a non-Catholic movement.)
Further reading (sedevacantists arguing for certain sacraments being invalid), see "Sacraments" section: https://traditionalmass.org/articles/
(A counter-argument might be that the "pope" has approved these "new sacraments", ergo they must be valid. The problem is that you can independently apply the criteria of determining sacramental validity and then end up probably more in doubt about if these sacraments are valid or not - my position. Consider if a formula was outright independently verifiable as "invalid" - could a pope approving the formula make it valid? This is kind of at the heart of the back and forth of considerations of authority versus the "facts" today - do certain "facts" demonstrate a person has no authority, or does a person's claim to authority seem to trump problems of ambiguity in this situation?)
