Philflies,
M2dude is right, and IIRC there is already a description of this much earlier in the thread.
The background of the 3/4 tab is, that
... on most aircraft you can - at the start of the take-off - 'run up the engines against the brakes', check they all deliver full power, and release the brakes only then.
... on Concorde it was impossible to 'hold the aircraft on the brakes' while going to full t/o thrust including the reheat (not so much because of insufficient brakes as insufficient 'footprint' of the wheels, IMHO).
... so, full t/o thrust (including reheat) didn't occur until the aircraft had already started the take-off roll.
If, at that point, one of the four reheats didn't light (which did happen at times), you did not have an awful lot of time to decide on whether you could continue 'on three' or had to reject the take-off.
Rather than having to check your pre-flight take-off calculations in a sheaf of papers or rely on your memory of the briefing, that little Heath Robinson "3/4 tab" gadget told you instantly whether to RTO NOW, or whether you could continue the take-off.
Sorry to repeat an old story, but Philflies asked the question, and not everybody has read the entire thread.....