Why it works
Even when White avoids 6.Bxb4?? and defends accurately, the move 4.e3?! still concedes the initiative. After 5...dxe3 6.fxe3 (otherwise the Lasker Trap fires) 6...Qh4+! 7.g3 Qe4 the queen lands in the centre with a double hit on the e3-pawn and the h1-rook. White must play 8.Nf3, and after 8...Qxe3+ Black regains the pawn with check. Material is equal, but White's king is stuck in the centre, the pawns are weak, and every black piece is active: the engine calls it level, yet it is unpleasant to play for White. The practical point: 4.e3 is not a one-move blunder but a strategic concession.
Refutation
4.Nf3! — sound development instead of 4.e3?!. White is in no hurry to deal with the d4-pawn, prepares g3/Bg2 and a3, and gets a normal game against the gambit pawn without any of these complications (the main line for White against the Albin).