Stimulated electron self-injection in the laser wakefield accelerator (LWFA) using density downramps is well known and regularly used to produce high energy electron bunches. The use of density gradients not only to stimulate injection but also control the properties of the injected electron bunch was recently presented by Tooley <i>et al</i>. [Phys. Rev. Lett. 119, 044801 (2017)], in which the authors put forward a model for controlling the velocity of the back of the bubble and compared to 2D and 3D particle-in-cell (PIC) data. This model is discussed and used to identify suitable LWFA parameters for ultra-short injection and repeated injection of multiple bunches. Quasi-3D PIC data is used to demonstrate injection of multiple bunches well separated in energy.