39Have you read about the +I effect properly?
Suppose there is a carbanion flanked by methyl groups. Obviously the negatively charged carbon is more electronegative than the uncharged carbons. This electronegativity difference causes the methyl groups to "release" their electrons into the carbanion.
Now what do we mean by "release"? See, we know from atomic structure theory that electrons are grouped in "electron clouds" where there is probability of finding them. The electron cloud of a methyl group is pulled toward the carbanion, distorted, because of the electronegativity difference(or polar effect, as it is called). Hence the "electron probability density", which is Ψ² (recall atomic orbital functions), increases as there is a larger area of electron cloud around the carbanion.
Note that the electrons remain in the same valence shell.