"Forces" was perhaps too strong a word!
My point was that against the normal 5 Bd3 the following are considered to be black's best defences:
1 d4 Nf6 2 c4 e6 3 Nf3 b6 4 e3 Bb7 5 Bd3 d5 followed by ...Bd6 e.g. 6 cxd5 exd5 7 O-O Bd6 as discussed by HgMan
1 d4 Nf6 2 c4 e6 3 Nf3 b6 4 e3 Bb7 5 Bd3 c5 6 O-O and now both 6...g6!? and the risky 6...Be7 7 Nc3 cxd4 (7...O-O?! 8 d5!) 8 exd4 d5 are theoretically doing well
1 d4 Nf6 2 c4 e6 3 Nf3 b6 4 e3 Bb7 5 Bd3 Bb4+ 6 Nbd2 (again 6 Nc3 transposes to the Rubinstein Nimzo). This is still a viable defence for black against 5 Nc3 of course.
The point is now that:
1 d4 Nf6 2 c4 e6 3 Nf3 b6 4 e3 Bb7 5 Nc3 d5 6 cxd5 exd5 7 Bb5+! c6 8 Bd3 is an improved version since the blunting of the Bb7 makes ...Ne4 harder to handle. Still not a complete disaster, but enough to favour white. 8...Bd6 9 O-O O-O 10 e4 or 9...Nbd7 10 e4 also score well.
1 d4 Nf6 2 c4 e6 3 Nf3 b6 4 e3 Bb7 5 Nc3 c5 is now just a good Benoni for white after 6 d5!, most likely transposing to the 7 O-O 8 d5! line in the first note above.
So white avoids black's best lines in the 4 e3 QID, although for me the 4 e3 b6 5 Nf3 Nimzo lines are not that great for white IMHO.
On a related note, in a NiC YB some years ago Igor Stohl sent a letter to the forum talking about how he used to favour the 5 Bd3 c5 6 0-0 g6 line but had been put off by 6 Nc3!? since 6...g6 7 d5! is good for white again. Yrjöla + Tella (and I think Wells also) give 6...cxd4 7 exd4 Bb4 as equalising, but I don't think either mentioned that 8 O-O O-O 9 Na4!? is another direct transposition to the Rubinstein Nimzo. (1 d4 Nf6 2 c4 e6 3 Nc3 Bb4 4 e3 b6 5 Bd3 Bb7 6 Nf3 O-O 7 O-O c5 8 Na4 cxd4 9 exd4).
Hope this helps and is of interest