What this hack does is limit the variation of Hot/Cold bonus in the ROM. In the vanilla ROM, the Hot/Cold can vary between -9/+8 (negative is Cold, positive is Hot).
The RNG function used by many things in the game starts at 0x01107A. The RNG function requires an "input", and at the end, produces a "random" result using the "input" as its negative limit. For the Hot/Cold bonus, this "input" value is stored at 0x0F70AD. In the vanilla ROM, the value is 09.
Changing the byte at 0

So using a value of 4 gives the following odds for hot/cold?
2/8 odds of -1 (random values -4 and -3)
5/8 odds of 0 (random values from -2 to +2)
1/8 odds of +1 (random value of +3)
And the +/-1 translates to +/-0.2 on the 0-6 scale (internally the game uses 0-30)