The development of equity did not stop centuries ago. The innovation of major principles has occurred several times in recent history. One example was discussed in Chapter 4 on Case Law: the case of Central London Property Trust v High Trees Ltd (1947). It holds that a person who is already in a contract, and who has made a promise by which he intentionally modifies his contractual rights against another party, will not be allowed to resile from such a promise.
Source: P131, How the Law Works, Gary Slapper
Would someone please explain the by? Why is there a preposition at all? Why not another preposition?
Why not use the relative pronoun THAT instead:
...and who has made a promise
by whichTHAT he intentionally modifies his contractual rights against another party...