The Tunnel of 5's is a combination of two moving averages:

5-period smoothed moving average applied to the highs
5-period smoothed moving average applied to the lows

(If your charting package doesn't have smoothed MAs, then use exponential)

The idea is that you want to be trading outside of this tunnel. The tunnel can also show you a "squeeze" prior to a break-out forming, which is part of the reason I do no need Bollinger Bands up on my chart (in fact, John Bollinger himself once told me that the best use of his bands is in option spread pricing, not in fx trading).

Many people "over trade" in that as soon as they get out of a long, they look to go short, and vice-versa. The Tunnel of 5's forces you to wait, to see if the reversal is really in fact a reversal. Sort of acts as a buffer zone. And, by the time price works it's way across and comes out the other side of this tunnel, chances are if you compare with your longer timeframe (multiple timeframes, don't forget), that will give you permission to change directions as well. The two work together quite nicely. You will find many moves accelerating once price moves to the outside of the Tunnel of 5's - this is because most bank traders are watching these levels, and consider a move "confirmed" (and jump on board themselves) once they see this.

The other function of the Tunnel of 5's is to distinguish between the end of a move and normal market "breathing". So if you're already in a trade, and price starts moving against you, the Tunnel of 5's can either give you the confidence to stay in longer (as long as candle bodies continue closing back outside of it), or tells you to perhaps consider abandoning your position early, ahead of target (if the Tunnel of 5's is breached with a candle body close).