When a clear-text variable references a secret variable, then anywhere that the clear-text variable gets referenced will show the secret value in clear text--for example, the Variables list for a build, the build log, etc. An example:

1) Define a secret variable, mySecretVar, and set it to "mysecret"
2) Define a clear text variable, myClearVar, and set it to "${vars.getValue("mySecretVar")}
3) After a build, myClearVar will show up in the Variables tab with its value in clear text, and possibly also in the build log depending on how it is used.