Type 2b muscle fibres are the ones sprinters want the most of.  They're the fastest of the two fast twitch fibres found in human muscle tissue.  The more you have, the faster and more powerfully your muscles work and the faster you can go.  Enduros don't want these at all, they're next to useless for endurance work.  Enduros spend their time trying to convert type two fibres to type one.  Type one, or 'slowtwitch' fibres are the ones that go all day, but 10 times slower in contraction speed than fast twitch fibres.

A very interesting article on 'Overshooting' is here.

The gist of it is :

a pattern of heavy resistance training followed by decreased activity causes first a decrease then an overshoot in the proportion of the fastest fibre type in the trained/detrained muscle group.

That's good!

a large increase in training volume for approximately three months will decrease the proportion of IIb fibres in the trained muscles; a subsequent reduction (not cessation) in training volume relative to the heavy resistance training phase should not only reverse this decrease but lead to a significant overshoot in the proportion of IIb fibres. In consequence, the potential for the rapid and forceful muscle contractions so crucial to sprint performance should be enhanced.
This conclusion is in line with the current training practices of many sprint athletes: a heavy resistance training phase followed by a taper in training volume and intensity in the lead up to the competitive season(9). And on the evidence of the Copenhagen research, others would be advised to follow their example, with three months of heavy resistance training followed by three months of relative detraining, with relatively reduced training volume in the run up to key targeted events.

Interesting.  One study found an increase from 9% to 18% (double!) the proportion of 2b fibres after a heavy block of training and then a layoff.  Ratios on their own are easily misinterpreted but that's a very significant increase.  It's rumoured that the British track sprint squad used this phenomenon to great effect in their preparation for Beijing.

Does this mean we should slack off before big sprint meetings?  It would seem so, at least in terms of heavy strength work.