Imagine a car that had 172 bhp and weighed 1080 kgs, ie, 1.08 tonnes. Oh, you've got one, well that's good. To get bhp/tonne (power/weight) you divide 172 by 1.08 and get 159.3. Ie, it has 159.3 bhp per tonne of weight. More bhp per tonne gives you better acceleration. The other way of doing power to weight is to determine how many kgs there are per bhp. To get kgs/bhp you just do your division the other way round, you divide 1080 by 172 and you get 6.28. Ie, it has 6.28 kgs per bhp. Less kgs per bhp gives you better acceleration.