Astable multivibrator diode improvement
The point of the diodes is that they stop conducting when the voltage on the collector rises when the transistor switches off.

In the first schematic, when Q1 opens the voltage on the collector of Q1 cannot rise immediately to V+ as C1 is charged to about -0.6 V when Q1 was conducting. So when Q1 switches off C1 wants to keep the collector of Q1 low. A current needs to flow through R1 to charge C1. This charge current causes a voltage drop across R1 and that prevents the voltage on the collector of Q1 to rise immediately to V+.

In the second schematic this works differently. As soon as TR1 switches off R1 can pull the collector to +9V. Now D1 prevents a charge current flowing through R1, instead all the charge current will need to come through R2. As no current will flow through R1 the voltage on the collector of TR1 can rise to +9V immediately.