We use this after strong or weak notrumps.
Once the penalty redouble is exposed, we are in a force at the 2-level only. Immediate doubles are penalty at any level.
Responder can show a 3-suiter short in clubs
by redoubling twice. 4333 hands are sometimes
shown this way. Responder can show a 3-suiter
short in diamonds via Rdbl..
In general, redoubles are takeout except after the business pass. They can be on hands that have psyched; if we want to play somewhere, we play it just doubled.
If opener bids an "impossible" suit, it's to play and natural, not fitted. Responder is not expected to correct. It's usually a six- or seven-card suit or a possible psych.
We will psych responses even after a double if we have a good place to run. If opener chooses a suit and responder overrides into a different suit, that suggests a psych. (It's unclear which action was the psych (both?) but opener cannot correct.)
We will once in awhile psych without a place to go, praying to find a non-doomed spot, especially against a good pair who we hope play takeout doubles. Down seven white undoubled in the 2-1 fit is probably better than being doubled if responder is broke.
We won't generally psych after a strong notrump, but are quite prone to it after a 10-12. Sometimes, confusion will be our epitaph. ...or escape.