What exactly is low bits subtraction
<p>I was reading <a href="http://www.gnu.org/software/guile/manual/html_node/Cheaper-Pairs.html#Cheaper-Pairs" rel="nofollow">this article</a> and thought that that everything was perfectly clear until I stumble upon this:</p>

<blockquote>
  <p>Again, most real Scheme systems use a slightly different implementation; for example, if <code>GET_PAIR</code> subtracts off the low bits of <code>x</code>, instead of masking them off, the optimizer will often be able to combine that subtraction with the addition of the offset of the structure member we are referencing, making a modified pointer as fast to use as an unmodified pointer.</p>
</blockquote>

<p>How exactly one can achieve this subtraction and how the optimizer will do its magic to modify the pointer as fast as unmodified pointer?</p>