When a trader takes a position in any binary put they are immediately exposed to possible adverse movements in time, volatility and the underlying. The risk of the latter can be immediately negated by taking an opposite position in the underlying equivalent to the delta of the position. For book-runners and market-makers hedging against an adverse movement in the underlying is of prime importance and hence the delta is the most widely used of the greeks.
For the same volatility the binary put option delta which is 50 ticks in-the-money is the same as the delta of the binary put 50 ticks out-of-the-money. In other words, the deltas are horizontally symmetric about the underlying when at-the-money.
Binary put option delta provide instant and easily understood information on the behaviour of the price of a binary put in relation to a change in the underlying. Binary puts always have negative deltas so an increase in the underlying causes a decrease in the value of the binary put.
2) When time to expiry is greater than 1 day (Figs.7a-c) the gearing of the binary put option is lower than the conventional put option, but when time to expiry is reduced (Figs.7d-e) the delta of the binary put becomes higher than the maximum value of 1.0 of the conventional put option.